ARM: exynos: clock: modify the set_mmc_clk for exynos4

Modified the mmc_set_clock for eynos4.
The goal of this patch is that fsys-div register should be reset.
And retore the div-value, not using the value of lowlevel_init.
(For using SDMMC4, this patch is needs)

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>