dt-bindings: phy: meson8b-usb2-phy: add support for the Meson8 SoCs

Meson8 uses the same USB PHY as found on the Meson8b and GXBB SoCs. This
adds a new compatible string to indicate that this binding also supports
the Meson8 SoCs.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>