ARM: S5P: Make the sleep code common for S5P series SoCs

The sleep code for S5PV210 and EXYNOS4 are identical; moreover
S5p64X0 and S5PC100 for which support will be added soon can
use the same procedure. Create a common sleep code in the plat-s5p
directory so that it can be re-used.

Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>