watchdog: JZ4740: Register a restart handler

The watchdog driver can restart the system by simply configuring the
hardware for a timeout of 0 seconds.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Cc: Wim Van Sebroeck <wim@linux-watchdog.org>
Cc: Mathieu Malaterre <malat@debian.org>
Cc: linux-watchdog@vger.kernel.org
Cc: linux-mips@linux-mips.org
Signed-off-by: James Hogan <jhogan@kernel.org>