watchdog: sp805: add restart handler

Add restart handler for SP805 watchdog so that the driver can be
used to reboot the system.

Signed-off-by: Jongsung Kim <neidhard.kim@lge.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>