ARM: S3C64XX: Eliminate plat-s3c64xx

Now we've move the support out of plat-s3c64xx for everything, eliminate
the platform directory arch/arm/plat-s3c64xx and remove it from the ARM
build configuration.

Note, PLAT_S3C64XX is kept around for the moment until the drivers that
depend on it can be updated, so it is moved to the mach-s3c64xx Kconfig.

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