ARM: s3c24xx: fix missing system reset

Commit f6361c6b ("ARM: S3C24XX: remove separate restart code")
removed usage of the watchdog reset platform code in favor of the
Samsung SoC watchdog driver.  However the latter was not selected thus
S3C24xx platforms lost reset abilities.

Cc: <stable@vger.kernel.org>
Fixes: f6361c6b

 ("ARM: S3C24XX: remove separate restart code")
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>