mfd: jz4740-adc: 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: Samuel Ortiz <sameo@linux.intel.com>