Documentation: dt: dwc3: add reference to the usb-xhci properties

dwc3 internally creates a usb-xhci device which means that all
properties documented in usb-xhci.txt are supported as well.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

Documentation/devicetree/bindings/usb/dwc3.txt

@@ -56,6 +56,10 @@ Optional properties:
 
  - <DEPRECATED> tx-fifo-resize: determines if the FIFO *has* to be reallocated.
 
+ - in addition all properties from usb-xhci.txt from the current directory are
+   supported as well
+
+
 This is usually a subnode to DWC3 glue to which it is connected.
 
 dwc3@4a030000 {