pata_imx: Convert to clk_prepare_enable/clk_disable_unprepare

With the new i.mx clock framework, we need to use
clk_prepare_enable/clk_disable_unprepare.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>