doc: usb: usbmisc-imx: add imx6sx compatible string

Add compatible string for imx6sx-usbmisc.
Signed-off-by: NPeter Chen <peter.chen@freescale.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -5,6 +5,7 @@ Required properties:
 - compatible: Should be one of below:
 	"fsl,imx6q-usbmisc" for imx6q
 	"fsl,vf610-usbmisc" for Vybrid vf610
+	"fsl,imx6sx-usbmisc" for imx6sx
 - reg: Should contain registers location and length
 
 Examples: