Documentation: DT: dma: Add Xilinx zynqmp dma device tree binding documentation

Device-tree binding documentation for Xilinx zynqmp dma engine
used in Zynq UltraScale+ MPSoC.

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