phy: phy-imx8mq-usb: Add USB PHY driver for i.MX8MQ

Add the USB PHY driver for i.MX8MQ to work with DWC3 USB controller.

Signed-off-by: Ye Li <ye.li@nxp.com>
Reviewed-by: Patrick Wildt <patrick@blueri.se>
Tested-by: Patrick Wildt <patrick@blueri.se>