doc: dt: mxs-phy: add compatible string for imx6sx-usbphy

Add compatible string for imx6sx-usbphy.
Signed-off-by: NPeter Chen <peter.chen@freescale.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>

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

@@ -5,6 +5,7 @@ Required properties:
 	* "fsl,imx23-usbphy" for imx23 and imx28
 	* "fsl,imx6q-usbphy" for imx6dq and imx6dl
 	* "fsl,imx6sl-usbphy" for imx6sl
+	* "fsl,imx6sx-usbphy" for imx6sx
   "fsl,imx23-usbphy" is still a fallback for other strings
 - reg: Should contain registers location and length
 - interrupts: Should contain phy interrupt