ath10k: debugfs file to enable Bluetooth coexistence feature

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>