dt-bindings: phy: meson8b-usb2: Add compatible string for Meson8m2

The USB2 PHY on Meson8m2 is identical to the one on Meson8b but
different to the one on Meson8. The only known difference is that Meson8
does not set the ACA_ENABLE bit while Meson8b and Meson8m2 do.
Add an explicit compatible string for Meson8m2 so those differences can
be taken care of.
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Tested-by: Thomas Graichen <thomas.graichen@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200512222424.549351-3-martin.blumenstingl@googlemail.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

Documentation/devicetree/bindings/phy/amlogic,meson8b-usb2-phy.yaml

@@ -16,6 +16,7 @@ properties:
           - enum:
               - amlogic,meson8-usb2-phy
               - amlogic,meson8b-usb2-phy
+              - amlogic,meson8m2-usb2-phy
           - const: amlogic,meson-mx-usb2-phy
       - const: amlogic,meson-gxbb-usb2-phy