Refactoring the pm.c to avoid using "soc_is_exynos" checks,
instead use the DT based lookup.

While at it, consolidate the common code across SoCs
and create static helper functions.
Signed-off-by: Vikas Sajjan <vikas.sajjan@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>