introduce xlnx-dpdma

This is the implementation of the DPDMA.

Signed-off-by: KONRAD Frederic <fred.konrad@greensocs.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Tested-By: Hyun Kwon <hyun.kwon@xilinx.com>
Message-id: 1465833014-21982-8-git-send-email-fred.konrad@greensocs.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>