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>