Documentation: DT: vdma: Update binding doc for multi-channel dma mode

This patch updates the device-tree binding doc for
AXI DMA multi channel dma mode.
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NKedareswara rao Appana <appanad@xilinx.com>
Signed-off-by: NVinod Koul <vinod.koul@intel.com>

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

@@ -40,6 +40,8 @@ Required properties for VDMA:
 Optional properties:
 - xlnx,include-sg: Tells configured for Scatter-mode in
 	the hardware.
+Optional properties for AXI DMA:
+- xlnx,mcdma: Tells whether configured for multi-channel mode in the hardware.
 Optional properties for VDMA:
 - xlnx,flush-fsync: Tells which channel to Flush on Frame sync.
 	It takes following values:
@@ -60,6 +62,8 @@ Optional child node properties:
 Optional child node properties for VDMA:
 - xlnx,genlock-mode: Tells Genlock synchronization is
 	enabled/disabled in hardware.
+Optional child node properties for AXI DMA:
+-dma-channels: Number of dma channels in child node.
 
 Example:
 ++++++++