ARM: S5P6440: Change the name for MMC Special Clock

Change the name of mmc spcial clock from mmc_bus to sclk_mmc to be
in line with the naming across the S5P SoCs

Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Signed-off-by: Sangbeom Kim <sbkim73@samsung.com>
[kgene.kim@samsung.com: minor edit of title]
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>