ARM: S3C24XX: remove separate restart code

The restart-handler series from Guenter Roeck got accepted recently and
implements among other things also the restart handler in the samsung
watchdog driver and where applicable in the clock drivers. So there is
no need for having the restart callbacks in s3c24xx boards anymore.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>