[media] s5p-mfc: Remove special clock rate management

The maximum rate of special clock depends on SoC variant and should
be set in device tree via assigned-clock-rates property, so remove
the code which forces special clock to 200MHz.

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>