exynos4-is: Add csis parent clock setup

exynos4-is: Add csis parent clock setup

With this patch the driver will set "parent" clock as a parent
clock of "mux" clock. When the samsung clocks driver is reworked
to use new composite clock type, the "mux" clock can be removed.

"parent" clock should be set in relevant dtsi file and can be
overwritten in a board dts file. This way it is ensured the
SCLK_CSIS has correct parent clock set, and the parent clock
can be selected per each board if required.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>