clk: samsung: exynos4: Fix definition of div_mmc_pre4 divider

The clock was missing CLK_SET_RATE_PARENT flag, which caused rate
setting failures due to inability of reconfiguration of second
divider behind it.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>