dt-bindings: phy-imx8mq-usb: add optional vbus supply regulator

Add a vbus supply regulator phandle, so the PHY can enable the VBUS
voltage rail when powering up.

Signed-off-by: Lucas Stach <[e-mail]>
Signed-off-by: Kishon Vijay Abraham I <[e-mail]>

Documentation/devicetree/bindings/phy/fsl,imx8mq-usb-phy.txt

···
 - clocks:	phandles to the clocks for each clock listed in clock-names
 - clock-names:	must contain "phy"
 
+Optional properties:
+- vbus-supply: A phandle to the regulator for USB VBUS.
+
 Example:
 	usb3_phy0: phy@381f0040 {
 		compatible = "fsl,imx8mq-usb-phy";