As not all QCA98XX radios are not connected to Bluetooth modules, enabling the
BT coex feature in firmware will have side effects if the radio's GPIO are
connected with other (non-BT) HW modules. Add debugfs file to control the
firmware BT coex logic and set the feature as disable by default to avoid that
btcoex is accidentally enabled.

To enable this feature, execute:

echo 1 > /sys/kernel/debug/ieee80211/phyX/ath10k/btcoex

To disable:

echo 0 > /sys/kernel/debug/ieee80211/phyX/ath10k/btcoex

The firmware support this feature since 10.2.4.54 on 2G-only board, dual band
or 5G boards don't support this. The feature's name is WMI_SERVICE_COEX_GPIO
and the btcoex file is not created if firmware doesn't support it.

Signed-off-by: Yanbo Li <yanbol@qca.qualcomm.com>
[kvalo@qca.qualcomm.com: use btcoex filename and other smaller fixes]
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>