ARM: EXYNOS: replace clock.c for other new EXYNOS SoCs

This patch changes the name of clock.c to clock-exynos4.c for other
EXYNOS series such as EXYNOS5. And since the header file of clock
is used only in arch/arm/mach-exynos, moves it in the local directory.

Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>