mac80211: ath9k: add GPIO support for AR9280 chip

Enable access to GPIO on Atheros wireless chip AR9280.
Support for 9280 is added to existing 9285/9287 subsystem
because these 3 chips differ only in number of GPIO pins.

Signed-off-by: Michal Cieslakiewicz <michal.cieslakiewicz@wp.pl>

SVN-Revision: 49251