dt-bindings: Document the Synopsys DW AXI DMA bindings

This patch adds documentation of device tree bindings for the Synopsys
DesignWare AXI DMA controller.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>