dma: Add Xilinx Zynq devcfg device model

dma: Add Xilinx Zynq devcfg device model

Add a minimal model for the devcfg device which is part of Zynq.
This model supports DMA capabilities and interrupt generation.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 83df49d8fa2d203a421ca71620809e4b04754e65.1467053537.git.alistair.francis@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>