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 <l.stach@pengutronix.de>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>