watchdog: s3c2410: add restart handler

On a lot of Samsung systems the watchdog is responsible for restarting the
system and until now this code was contained in plat-samsung/watchdog-reset.c.

With the introduction of the restart handlers, this code can now move into
driver itself, removing the need for arch-specific code.

Tested on a S3C2442 based GTA02

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>