ARM: S3C2443: Move parts of the clock code to common clock file

To share code with some of the newer parts such as the S3C2416, move
parts of arch/arm/mach-s3c2443/clock.c to a common file called
arch/arm/plat-s3c24xx/s3c2443-clock.c.

Update the build configuration to deal with this new file.

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