dt-bindings: phy: uniphier-usb3ss: Treat vbus-supply as optional

The usb-device doesn't need "vbus-supply" property, so the property should
be removed from required.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/1648637715-19262-3-git-send-email-hayashi.kunihiko@socionext.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>