watchdog: mtk_wdt: add suspend/resume support

add mediatek watchdog driver suspend/resume support

Signed-off-by: Greta Zhang <greta.zhang@mediatek.com>
Signed-off-by: Roger Lu <roger.lu@mediatek.com>
Signed-off-by: Eddie Huang <eddie.huang@mediatek.com>
Acked-by: Matthias Brugger <matthias.bgg@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>