ARM: EXYNOS: Add support for mapping PMU base address via DT

Add support for mapping Samsung Power Management Unit (PMU)
base address from device tree.

Signed-off-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>