ARM64: dts: meson-gxm: add GXM specific USB host configuration

The USB configuration on GXM is slightly different than on GXL. The dwc3
controller's internal hub has three USB2 ports (instead of 2 on GXL)
along with a dedicated USB2 PHY for this port. However, it seems that
there are no pins on GXM which would allow connecting the third port to
a physical USB port.
Passing the third PHY is required though, because without it none of the
other USB ports is working (this seems to be a limitation of how the
internal USB hub works, if one PHY is disabled then no USB port works).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>