Exynos: clock: support get_mmc_clk for exynos

To get exactly clock value for mmc, support the get_mmc_clk() like
set_mmc_clk().

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>