ARM: S3C64XX: Move headers into machine include directory

Move the register and GPIO definition files from plat-s3c64xx into the
machine include direcotry as they are unlikely to be reused outside
mach-s3c64xx.

This move includes removing the empty <mach/regs-clock.h> and replacing
it with the <plat/regs-clock.h> implementation.

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