wcn36xx: Add support for Factory Test Mode (FTM)

Introduce infrastructure for supporting Factory Test Mode (FTM) of the
wireless LAN subsystem. In order for the user space to access the
firmware in test mode the relevant netlink channel needs to be exposed
from the kernel driver.

The above is achieved as follows:
1) Register wcn36xx driver to testmode callback from netlink
2) Add testmode callback implementation to handle incoming FTM commands
3) Add FTM command packet structure
4) Add handling for GET_BUILD_RELEASE_NUMBER (msgid=0x32A2)
5) Add generic handling for all PTT_MSG packets

Signed-off-by: Eyal Ilsar <eilsar@codeaurora.org>
Signed-off-by: Ramon Fried <ramon.fried@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>