ARM: EXYNOS: Get current parent clock for power domain on/off

Using a fixed (by DTS) parent for clocks when turning on the power
domain may introduce issues in other drivers. For example when such
driver changes the parent during runtime and expects that he is the
only place of such change.

Do not rely on DTS providing the fixed parent for such clocks. Instead
before switching domain off, grab a current parent of a clock with
clk_get_parent().

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Tested-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Kukjin Kim <kgene@kernel.org>