Documentation: DT: vdma: update binding doc for AXI DMA

This patch updates the device-tree binding doc for
adding support for AXI DMA.
Also this patch differentiates required properties b/w
DMA and VDMA.

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