clk: imx: imx8mm: add set_parent callback

Add set_parent callback, then assigned-clock-parents in dts could
be work.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Tested-by: Frieder Schrempf <frieder.schrempf@kontron.de>