clk: imx: pfdv2: switch to use determine_rate

Per clk_ops, compared with round_rate, determine_rate could optionally
support the parent clock that should be used to provide the clock rate.

In this patch, the parent clock is just parent->rate as round_rate.

The following patch will calculate the best parent clock.

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