devicetree: Add bindings documentation for Analog Devices AXI-DMAC

Add the devicetree descriptor for the Analog Devices AXI-DMAC DMA
controller. This is a soft peripheral used in FPGAs and the bindings
describe how it is connected to the system (clock, interrupt, memory map)
as well as the configuration options that were used when the peripheral was
instantiated.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>