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.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>