arm64: dts: meson-g12a: add missing dwc2 phy-names

The G12A USB2 OTG capable PHY uses a 8bit large UTMI bus, and the OTG
controller gets the PHY but width by probing the associated phy.

By default it will use 16bit wide settings if a phy is not specified,
in our case we specified the phy, but not the phy-names.

The dwc2 bindings specifies that if phys is present, phy-names shall be
"usb2-phy".

Adding phy-names = "usb2-phy" solves the OTG PHY bus configuration.

Fixes: 9baf7d6b ("arm64: dts: meson: g12a: Add G12A USB nodes")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>