dma: Add Xilinx AXI DMA DT Binding Documentation

Device-tree binding documentation of Xilinx DMA Engine

Signed-off-by: Srikanth Thokala <sthokal@xilinx.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>