dt-bindings: phy: meson8b-usb2: Convert to json-schema

dt-bindings: phy: meson8b-usb2: Convert to json-schema

Now that we have the DT validation in place, let's convert the device
tree bindings for the Amlogic Meson8, Meson8b, Meson8m2 and GXBB USB2
PHY over to a YAML schema.

While here, also add the fallback compatible string
"amlogic,meson-gxbb-usb2-phy" which is already used in
arch/arm/boot/dts/meson{,8,8b}.dtsi.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Tested-by: Thomas Graichen <thomas.graichen@gmail.com>
Link: https://lore.kernel.org/r/20200512222424.549351-2-martin.blumenstingl@googlemail.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>