ar71xx: add an external reset callback for ar913x and use it in ath9k
should reduce the frequency of DMA stop failures

Backport of r36530

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt@36664 3c298f89-4303-0410-b956-a3cf2f4a3e73