ARM: mxs: convert platform code to clk_prepare/clk_unprepare

The patch converts mxs platform code to clk_prepare/clk_unprepare
by using helper functions clk_prepare_enable/clk_disable_unprepare.

Signed-off-by: Richard Zhao <richard.zhao@linaro.org>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>