dma: xilinx: Use of_dma framework in dma test client

This patch does following things:
- Uses of_dma framework in dma test client.
- Document the device node for dma test client.

Signed-off-by: Kedareswara rao Appana <appanad@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>