ARM: S3C64XX: Move if ARCH_S3C64XX statement into mach-s3c64xx

All other platforms have this condition checked inside their own Kconfig
files, so for consistency this patch makes it this way for mach-s3c64xx
as well.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>