doc: usb: usbmisc-imx: add imx7d compatible string

Add compatible string for imx7d-usbmisc.
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Li Jun <jun.li@nxp.com>
Signed-off-by: Peter Chen <peter.chen@nxp.com>

Documentation/devicetree/bindings/usb/usbmisc-imx.txt

@@ -6,6 +6,7 @@ Required properties:
 	"fsl,imx6q-usbmisc" for imx6q
 	"fsl,vf610-usbmisc" for Vybrid vf610
 	"fsl,imx6sx-usbmisc" for imx6sx
+	"fsl,imx7d-usbmisc" for imx7d
 - reg: Should contain registers location and length
 
 Examples: