watchdog: mtk_wdt: add wdt shutdown callback to disable wdt if enabled

Without .shutdown(), watchdog might reset the system during power off.
For example, if watchdog's timeout is set to 30s, then it is reset to
zero by mtk_wdt_ping(). During power off, no app will ping watchdog,
but watchdog is still running and may trigger reset.

Signed-off-by: Greta Zhang <greta.zhang@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>