MIPS: ath79: add common watchdog device

All supported SoCs have a built-in hardware watchdog driver. This patch
registers a platform_device for that to make it usable.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Imre Kaloz <kaloz@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: Luis R. Rodriguez <lrodriguez@atheros.com>
Cc: Cliff Holden <Cliff.Holden@Atheros.com>
Cc: Kathy Giori <Kathy.Giori@Atheros.com>
Patchwork: https://patchwork.linux-mips.org/patch/1955/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>