introduce xlnx-dp

This is the implementation of the DisplayPort.
It has an aux-bus to access dpcd and edid.

Graphic plane is connected to the channel 3.
Video plane is connected to the channel 0.
Audio stream are connected to the channels 4 and 5.

Signed-off-by: KONRAD Frederic <fred.konrad@greensocs.com>
Tested-By: Hyun Kwon <hyun.kwon@xilinx.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 1465833014-21982-9-git-send-email-fred.konrad@greensocs.com
[PMM: fixed format strings]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>