arm64: zynqmp: Enable xlnx,zynqmp-dwc3 driver for xilinx boards

The commit 84770f02 ("usb: dwc3: Add driver for Xilinx platforms")
finally add proper support for Xilinx dwc3 driver. This patch is adding DT
description for it.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Michael Tretter <m.tretter@pengutronix.de>
Link: https://lore.kernel.org/r/640a3bc0dc3e32560d3e84c2f78b5ae561396eb0.1628244703.git.michal.simek@xilinx.com