exynos4-is: Add fimc parent clock setup

exynos4-is: Add fimc 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 related dtsi file and can be
overwritten in a board dts file. This way it is ensured the
SCLK_FIMC clock 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>