dt-bindings: dmaengine: xilinx_dma: Add MCMDA channel ID index description

MCDMA IP provides up to 16 multiple channels of data movement each on
MM2S and S2MM paths. Inline with implementation, in the binding add
description for the channel ID start index and mention that it's fixed
irrespective of the MCDMA IP configuration(number of read/write channels).
Signed-off-by: Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1649939061-6675-1-git-send-email-radhey.shyam.pandey@xilinx.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

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

@@ -110,7 +110,11 @@ axi_vdma_0: axivdma@40030000 {
 Required properties:
 - dmas: a list of <[Video DMA device phandle] [Channel ID]> pairs,
 	where Channel ID is '0' for write/tx and '1' for read/rx
-	channel.
+	channel. For MCMDA, MM2S channel(write/tx) ID start from
+	'0' and is in [0-15] range. S2MM channel(read/rx) ID start
+	from '16' and is in [16-31] range. These channels ID are
+	fixed irrespective of IP configuration.
+
 - dma-names: a list of DMA channel names, one per "dmas" entry
 
 Example: