dts: pinctrl: unify pinctrl binding for imx8mp/n/m

Currently imx8mm/n use pinctrl binding of nxp,imx8m-pinctrl.yaml which
is used for imx8mq series platforms, but imx8mm/n have different pinctrl
hardware module with imx8mq, so change imx8mm/n to use binding of
nxp,imx8mp-pinctrl.yaml as imx8mm, imx8mn and imx8mp have the same
pinctrl hardware module.

Signed-off-by: Jiafei Pan <Jiafei.Pan@nxp.com>