watchdog: bcm47xx_wdt.c: add restart handler support

Just like in case of other watchdog drivers, use the new kernel core
API to provide restart support.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>