vdma: xilinx: Use of_dma framework in vdma test client

vdma: xilinx: Use of_dma framework in vdma test client

This patch does following things:
- Uses of_dma framework in vdma test client
- Document the device node for vdma test client
- Add the above node as an example to existing vdma
  device binding documentation

Signed-off-by: Srikanth Thokala <sthokal@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>