Got Reef (Apollo Lake?) running with QCA6174 and ar10k. 
Smoke-tests: Booted, ran speedtests on GoogleGuest, could hit 160mbps, suspend-resume and check wifi comes back on properly.
Proper tests: wifi_matfunc and wifi_perf in progress. 

Log proof: 

# cd /var/log
localhost log # dmesg | grep ath
[    1.047186] Unsafe core_pattern used with suid_dumpable=2. Pipe handler or fully qualified core dump path required.
[   14.281696] ath10k_pci 0000:01:00.0: pci irq msi interrupts 1 irq_mode 0 reset_mode 0
[   14.728470] ath10k_pci 0000:01:00.0: Direct firmware load for ath10k/cal-pci-0000:01:00.0.bin failed with error -2
[   14.732231] ath10k_pci 0000:01:00.0: Direct firmware load for ath10k/QCA6174/hw3.0/firmware-5.bin failed with error -2
[   14.732247] ath10k_pci 0000:01:00.0: could not fetch firmware file 'ath10k/QCA6174/hw3.0/firmware-5.bin': -2
[   14.737553] ath10k_pci 0000:01:00.0: qca6174 hw3.2 target 0x05030000 chip_id 0x00340aff sub 168c:3361
[   14.737564] ath10k_pci 0000:01:00.0: kconfig debug 1 debugfs 1 tracing 1 dfs 0 testmode 1
[   14.741160] ath10k_pci 0000:01:00.0: firmware ver WLAN.RM.2.0-00180-QCARMSWPZ-1 api 4 features wowlan,ignore-otp,no-4addr-pad crc32 75dee6c5
[   14.860686] ath10k_pci 0000:01:00.0: board_file api 2 bmi_id N/A crc32 46b57247
[   17.299649] ath10k_pci 0000:01:00.0: htt-ver 3.26 wmi-op 4 htt-op 3 cal otp max-sta 32 raw 0 hwcrypto 1
[   17.381134] ath: EEPROM regdomain: 0x0
[   17.381142] ath: EEPROM indicates default country code should be used
[   17.381144] ath: doing EEPROM country->regdmn map search
[   17.381146] ath: country maps to regdmn code: 0x3a
[   17.381148] ath: Country alpha2 being used: US
[   17.381149] ath: Regpair used: 0x3a

localhost log # cd /sys/module/
localhost module # cd ar10k_mac80211/
localhost ar10k_mac80211 # ls
coresize  holders  initsize  initstate  notes  parameters  refcnt  sections  taint  uevent
localhost ar10k_mac80211 # cd parameters/
localhost parameters # ls
beacon_loss_count  ieee80211_default_rc_algo  max_nullfunc_tries  max_probe_tries  minstrel_vht_only  probe_wait_ms
localhost parameters # cat beacon_loss_count 
7

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/third_party/kernel/+/99a12a3296189a77073f0df6fafcc365008e64a5

commit 99a12a3296189a77073f0df6fafcc365008e64a5
Author: Guenter Roeck <groeck@chromium.org>
Date: Tue Mar 07 06:24:28 2017

FIXUP: CHROMIUM: Add initial version of ar10k wifi drv (take 2)

Building x86_64:allyesconfig results in the following build warnings.

WARNING: drivers/net/wireless/ar10k/ath/ath10k/ath10k_core:
	'ath10k_print_driver_info' exported twice.
	Previous export was in vmlinux
WARNING: drivers/net/wireless/ar10k/ath/ath10k/ath10k_core:
	'ath10k_core_create' exported twice.  Previous export was in vmlinux
WARNING: drivers/net/wireless/ar10k/ath/ath10k/ath10k_core:
	'ath10k_dbg' exported twice. Previous export was in vmlinux
WARNING: drivers/net/wireless/ar10k/ath/ath10k/ath10k_core:
	'ath10k_core_stop' exported twice. Previous export was in vmlinux

... and many more.

Disable the 'chromeos' version of the driver if the upstream version
is enabled to avoid unpredictable behavior. This effectivelt disables
the chromeos version for test builds (allmodconfig, allyesconfig),
which is intentional since the chromeos version is presumably built
and runtime tested as part of the regular build process.

BUG= chromium:686507 
TEST=Build x86_64:allyesconfig and observe lack of build warnings

Change-Id: Ia348dab454c03dc99e079cf82f4c1f92fa2b51f0
Signed-off-by: Guenter Roeck <groeck@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/449068
Reviewed-by: Kirtika Ruchandani <kirtika@chromium.org>

[modify] https://crrev.com/99a12a3296189a77073f0df6fafcc365008e64a5/drivers/net/wireless/ar10k/Kconfig

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/third_party/kernel/+/0e039cf2bb35302d312c65064a617a2a05754f00

commit 0e039cf2bb35302d312c65064a617a2a05754f00
Author: Guenter Roeck <groeck@chromium.org>
Date: Wed May 09 21:40:20 2018

FIXUP: CHROMIUM: Add initial version of ar10k wifi drv (take 3)

Test builds (allmodconfig) randomly fail with the following error.

ERROR: "ath10k_htt_rx_pktlog_completion_handler" [drivers/net/wireless/ar10k/ath/ath10k/ath10k_pci.ko] undefined!

This can only happen if both ATH10K and AR10K are built and interfer
with each other. Analysis shows that "depends on !ATH10K" is wrong.
It needs to be "depends on ATH10K=n".

BUG= chromium:686507 
TEST=Build x86_64:allmodconfig and observe lack of build errors

Change-Id: I16ea22ff5bfa614660ce4fa57bc743bffae37b7f
Signed-off-by: Guenter Roeck <groeck@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1033486
Reviewed-by: Kirtika Ruchandani <kirtika@chromium.org>

[modify] https://crrev.com/0e039cf2bb35302d312c65064a617a2a05754f00/drivers/net/wireless/ar10k/Kconfig