ARM: EXYNOS: change the prefix S5P_ to EXYNOS4_ for clock

This patch changes prefix of the clk register from S5P_ to
EXYNOS4_ for new EXYNOS SoCs such as EXYNOS5 and adds prefix
exynos4_ on clk declarations.

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