[ARM] 4161/1: S3C24XX: fix leakage of plat-s3c24xx Kconfig items

Do not export S3C24XX from plat-s3c24xx on non-s3c24xx systems

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>