ARM: EXYNOS4: Support system level power down configuration

EXYNOS4 supports 3 different system level power down mode by PMU (Power
Management Unit). Each power down mode need to configure many PMU registers
with different value. This patch supports function to configure PMU registers
with pre-defined values in PMU code. This function may be used by PM code and
AFTR/LPA support driver.

Signed-off-by: Jaecheol Lee <jc.lee@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>