ath9k_hw: add support for the AR9003 baseband watchdog

The baseband watchdog will monitor blocks of the baseband
through timers and will issue an interrupt when things are
detected to be stalled. It is only available on the AR9003
family.

Cc: Sam Ng <sam.ng@atheros.com>
Cc: Paul Shaw <paul.shaw@atheros.com>
Cc: Don Breslin <don.breslin@atheros.com>
Cc: Cliff Holden <cliff.holden@atheros.com
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>