[media] s5p-mfc: Rework clock handling

This patch changes the code for handling clocks. Now clocks are defined
per each device variant, what is a preparation for adding support for
Exynos 5433 MFC V8, which has more clocks than all previous versions.
Also use devm_clk_get() to simplify cleanup path.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>