ARM: dts: add missing clock to MFC device for exynos4

sclk_mfc is required for MFC device since commit
0c227217 ("media: s5p-mfc: rename
special clock to sclk_mfc"), so add it to exynos4 dts.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>