Documentation: bindings: add doc for zynq USB

Document the binding for the zynq specific chipidea UDC binding.
Signed-off-by: Nathan Sullivan <nathan.sullivan@ni.com>
Signed-off-by: Peter Chen <peter.chen@freescale.com>

Documentation/devicetree/bindings/usb/ci-hdrc-usb2.txt

@@ -6,6 +6,7 @@ Required properties:
 	"lsi,zevio-usb"
 	"qcom,ci-hdrc"
 	"chipidea,usb2"
+	"xlnx,zynq-usb-2.20a"
 - reg: base address and length of the registers
 - interrupts: interrupt for the USB controller