ARM: EXYNOS: read initial state of power domain from hw registers

Some bootloaders disable unused power domains to reduce power
consuption. Power domain driver can easily read the actual state from
the hardware registers instead of assuming that their initial state is
always 'on'.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>