dt-bindings: phy: add binding for Freescale i.MX8MQ USB3 PHY

This adds the binding for the USB3 PHY found on the i.MX8M SoC.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>