dt-bindings: phy: meson-g12a-usb2-phy: convert to yaml

Now that we have the DT validation in place, let's convert the device tree
bindings for the Amlogic G12A USB2 PHY over to a YAML schemas.

While the original phy bindings specifies phy-supply as required,
the examples and implementations makes it optional, thus phy-supply
is not in the required list of attributes.

Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Rob Herring <robh@kernel.org>