Documentation: devicetree: Fix Xilinx VDMA specification

The specification requires xlnx,data-width, but example and driver use
xlnx,datawidth. Change the specification to match the implementation.

Reviewed-by: Michal Simek <michal.simek@xilinx.com>
Fixes: eebeac03

 ("dma: Add Xilinx Video DMA DT Binding Documentation")
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>