ARM: dts: imx8mn: Replace deprecated fsl,usbphy DT props with phys

ARM: dts: imx8mn: Replace deprecated fsl,usbphy DT props with phys

The fsl,usbphy DT property is deprecated, replace it with phys DT
property and specify #phy-cells, so that the generic PHY framework
can parse the PHY bindings without any extra hacking.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Ye Li <ye.li@nxp.com>
Cc: uboot-imx <uboot-imx@nxp.com>