ARM: EXYNOS: add support for sub-power domains

This patch adds support for making one power domain a sub-domain of
other domain. This is useful for modeling power dependences for devices
like TV Mixer or Camera ISP, which needs to have more than one power
domain enabled to be operational.

Based on previous work by Amit Daniel Kachhap <amit.daniel@samsung.com>.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Kukjin Kim <kgene@kernel.org>