ARM: EXYNOS: Add PMU support for exynos5420

Adds initial PMU settings for exynos5420. This is required for
future S2R and Switching support.

Signed-off-by: Thomas Abraham <thomas.ab@samsung.com>
Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Signed-off-by: Vikas Sajjan <vikas.sajjan@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>