dt-bindings: phy: uniphier-usb2: Add vbus-supply

UniPhier USB2 PHY controller can apply vbus-supply.
Add "vbus-supply" property to fix the following warning.

  uniphier-pro4-ace.dtb: usb-glue: phy@2: 'vbus-supply' does not match any of the regexes: 'pinctrl-[0-9]+'
      From schema: Documentation/devicetree/bindings/phy/socionext,uniphier-usb2-phy.yaml

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/1648637715-19262-2-git-send-email-hayashi.kunihiko@socionext.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>