clk: samsung: exynos5250: Add MDMA0 clocks

Adds gate clock for MDMA0 on Exynos5250 SoC. This is needed to ensure
that the clock is enabled when MDMA0 is used on systems on which
firmware gates the clockby default.

Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Acked-by: Mike Turquette <mturquette@linaro.org>
[t.figa: Updated patch description.]
Signed-off-by: Tomasz Figa <t.figa@samsung.com>