ARM: S3C64XX: Remove plat-s3c64xx Kconfig and PLAT_S3C64XX

Remove the Kconfig and PLAT_S3C64XX defines for the previous S3C64XX
directory structure now that the code is moved into mach-s3c64xx.

Note, we cannot currently remove plat-s3c64xx directory as we have a
pair of include files used within plat-s3c and plat-samsung that need
to find a new home.

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