ath9k: Add a debugfs interface to dump chip registers

/<debugfs_root>/ieee80211/phyX/ath9k/regdump is the interface
to dump the registers.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>