watchdog: dw_wdt: add restart handler support

The kernel core now provides an API to trigger a system restart.
Register with it to support restarting the system via. watchdog.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>