ARM: S3C64XX: Select CPU_V6K instead of CPU_V6

All S3C64XX SoCs come with ARM1176JZF-s core, which fully supports
ARMv6K extensions. This patch lets the kernel use them on S3C6410 by
adding selection of CPU_V6K to ARCH_S3C64XX.
To use V6K extensions, these symbols must not be selected at the same
time, so this patch keeps only CPU_V6K selected for S3C64XX.

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