EXYNOS5420: Add power register structure.

Add structure for power register for Exynos5420
Signed-off-by: NRajeshwari S Shinde <rajeshwari.s@samsung.com>
Acked-by: NSimon Glass <sjg@chromium.org>
Signed-off-by: NMinkyu Kang <mk7.kang@samsung.com>