ath10k: add WMI_SERVICE_AVAILABLE_EVENT support

Add WMI_SERVICE_AVAILABLE_EVENT to extend WMI_SERVICE_READY_EVENT,
the 128bit service map in WMI_SERVICE_READY_EVENT is not enough
for firmware to notice new WLAN service to host driver. Hereby,
for thoese new WLAN service, firmware will notice host driver by
WMI_SERVICE_AVAILABLE_EVENT.

Signed-off-by: Alan Liu <alanliu@codeaurora.org>
Signed-off-by: Carl Huang <cjhuang@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>