ARM: SAMSUNG: Move clock.c to arch/arm/plat-samsung

This is the core implementation of the clock code for all Samsung based
SoCs, so move it to arch/arm/plat-samsung (the clock.h file has already
been moved).

Since the file is built for every Samsung SoC, no changes are needed to the
Kconfig system.

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