arm64: dts: imx8m: 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. No functional change.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP Linux Team <linux-imx@nxp.com>
Cc: Shawn Guo <shawnguo@kernel.org>
To: linux-arm-kernel@lists.infradead.org
Signed-off-by: Shawn Guo <shawnguo@kernel.org>