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.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

Documentation/devicetree/bindings/phy/socionext,uniphier-usb3ss-phy.yaml

@@ -61,7 +61,7 @@ properties:
           - const: phy
 
   vbus-supply:
-    description: A phandle to the regulator for USB VBUS
+    description: A phandle to the regulator for USB VBUS, only for USB host
 
 required:
   - compatible
@@ -71,7 +71,6 @@ required:
   - clock-names
   - resets
   - reset-names
-  - vbus-supply
 
 additionalProperties: false