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 <[e-mail]>
Fixes: eebeac03db93 ("dma: Add Xilinx Video DMA DT Binding Documentation")
Signed-off-by: Andreas Färber <[e-mail]>
Reviewed-by: Soren Brinkmann <[e-mail]>
Signed-off-by: Vinod Koul <[e-mail]>

Documentation/devicetree/bindings/dma/xilinx/xilinx_vdma.txt

···
 - compatible: It should be either "xlnx,axi-vdma-mm2s-channel" or
 	"xlnx,axi-vdma-s2mm-channel".
 - interrupts: Should contain per channel VDMA interrupts.
-- xlnx,data-width: Should contain the stream data width, take values
+- xlnx,datawidth: Should contain the stream data width, take values
 	{32,64...1024}.
 
 Optional child node properties: