ARM: S3C64XX: Merge s3c6400-clock.c and clock.c into mach-s3c64xx

Merge plat-s3c64xx/s3c6400-clock.c mach-s3c64xx/clock.c placing all the
clock code into one place.

Note, no effort is made in this patch to squash the init functions
together.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>