[WATCHDOG] s3c2410 watchdog - replace reboot notifier

Patch from Dimitry Andric <dimitry.andric@tomtom.com>

Change to using platfrom driver's .shutdown method instead
of an reboot notifier

Signed-off-by: Dimitry Andric <dimitry.andric@tomtom.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>