arm64: dts: imx8mm: Move usbphy out of soc node

usbphy nodes do not have any register properties and thus
shouldn't be placed inside the bus.

Move usbphy nodes from soc node to root node in order to fix
the following build warnings with W=1:

arch/arm64/boot/dts/freescale/imx8mm.dtsi:681.27-687.6: Warning (simple_bus_reg): /soc/bus@32c00000/usbphynop1: missing or empty reg/ranges property
arch/arm64/boot/dts/freescale/imx8mm.dtsi:710.27-716.6: Warning (simple_bus_reg): /soc/bus@32c00000/usbphynop2: missing or empty reg/ranges property

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>