ARM: S3C64XX: Merge mach-s3c6400 and mach-s3c6410

As per discussions with Russell King on linux-arm-kernel, it appears that
both mach-s3c6400 and mach-s3c6410 are so close together that they should
simply be merged into mach-s3c64xx.

Note, this patch does not eliminate any of the bits that are still common,
it is simply a move of the two directories together, any further common
code will be eliminated or moved in further patches.

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