ARM: S3C24XX: move plat-s3c24xx/cpu.c

Following the model of s3c64xx cpu.c becomes common.c in mach-s3c24xx,
to got more common code added later on.

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