ARM: mxs: select HAVE_CLK_PREPARE for clock

This patch adds clk_prepare/clk_unprepare for mxs clock api by
renaming the existing non-atomic clk_enable/clk_disable to
clk_prepare/clk_unprepare and adding a pair of dummy
clk_enable/clk_disable.  Then with selecting HAVE_CLK_PREPARE for
mxs clock, we can fix the mutex locking warning that has been
reported for a few times.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 776d76b..dc46163 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -442,6 +442,7 @@
 	select ARCH_REQUIRE_GPIOLIB
 	select CLKDEV_LOOKUP
 	select CLKSRC_MMIO
+	select HAVE_CLK_PREPARE
 	help
 	  Support for Freescale MXS-based family of processors