dt-bindings: usb: dwc3-xilinx: Add documentation for Versal DWC3 Controller

Add documentation for Versal DWC3 controller. Add required property
'reg' for the same. Also add optional properties for snps,dwc3.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Manish Narani <manish.narani@xilinx.com>
Link: https://lore.kernel.org/r/1615963949-75320-2-git-send-email-manish.narani@xilinx.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>