[media] media: s5p-mfc: rename special clock to sclk_mfc

Commit d19f405a ("[media] s5p-mfc: Fix
selective sclk_mfc init") added support for special clock handling
(named "sclk-mfc"). However this clock is not defined yet on any
platform, so before adding it to all Exynos platform, better rename it
to "sclk_mfc" to match the scheme used for all other special clocks on
Exynos platform.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>