clk: imx: pllv4: use prepare/unprepare

It is not good to use enable/disable for PLLv4 which needs time to
lock, because enable/disable is expected to be able run in
interrupt context. So use prepare/unprepare.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>