watchdog: Add driver for Mediatek watchdog

This patch adds a driver for the Mediatek SoC integrated
watchdog. This driver supports watchdog and software reset
for mt65xx and mt81xx SoCs.

Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Tested-by: Eddie Huang <eddie.huang@mediatek.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>