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.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>