watchdog: add driver for the Atheros AR71XX/AR724X/AR913X SoCs

This patch adds a driver for the built-in hardware watchdog device
of the Atheros AR71XX/AR724X/AR913X SoCs.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Imre Kaloz <kaloz@openwrt.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>