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: Rob Herring <robh@kernel.org>
Signed-off-by: Kedareswara rao Appana <appanad@xilinx.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>