doc: dt-binding: mxs-usb-phy: add compatible for 7ulp

Add compatible for 7ulp USB PHY.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

Documentation/devicetree/bindings/phy/mxs-usb-phy.txt

@@ -7,6 +7,7 @@ Required properties:
 	* "fsl,imx6sl-usbphy" for imx6sl
 	* "fsl,vf610-usbphy" for Vybrid vf610
 	* "fsl,imx6sx-usbphy" for imx6sx
+	* "fsl,imx7ulp-usbphy" for imx7ulp
   "fsl,imx23-usbphy" is still a fallback for other strings
 - reg: Should contain registers location and length
 - interrupts: Should contain phy interrupt
@@ -23,7 +24,7 @@ Optional properties:
   the 17.78mA TX reference current. Default: 100
 
 Example:
-usbphy1: usbphy@20c9000 {
+usbphy1: usb-phy@20c9000 {
 	compatible = "fsl,imx6q-usbphy", "fsl,imx23-usbphy";
 	reg = <0x020c9000 0x1000>;
 	interrupts = <0 44 0x04>;