mmc: jz4740: Use clk_prepare_enable/clk_disable_unprepare

In preparation to switching the jz4740 clk driver to the
common clk framework, update the clk enable/disable calls
to clk_prepare_enable/clk_disable_unprepare.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Chris Ball <cjb@laptop.org>