mmc: sdhci-spear: Add clk_{un}prepare() support

clk_{un}prepare is mandatory for platforms using common clock framework.
Since this driver is used by SPEAr platform, which supports common clock
framework, add clk_{un}prepare() support for it.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>