dt-bindings: phy-imx8mq-usb: add compatible string for imx8mp usb phy

Add "fsl,imx8mp-usb-phy" compatible string for imx8mp usb phy, which is
similar with imx8mq usb phy but with some different customizations.
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Li Jun <jun.li@nxp.com>
Link: https://lore.kernel.org/r/1598276014-2377-1-git-send-email-jun.li@nxp.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

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

 * Freescale i.MX8MQ USB3 PHY binding
 
 Required properties:
-- compatible:	Should be "fsl,imx8mq-usb-phy"
+- compatible:	Should be "fsl,imx8mq-usb-phy" or "fsl,imx8mp-usb-phy"
 - #phys-cells:	must be 0 (see phy-bindings.txt in this directory)
 - reg:		The base address and length of the registers
 - clocks:	phandles to the clocks for each clock listed in clock-names