ARM: SAMSUNG: Move S3C6400 PLL code to <plat/pll.h> for re-use

The S3C6400 EPLL code matches the S3C2416 and compatible SoCs, so move
it from mach-s3c64xx into <plat/pll.h> for easy reuse.

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