dt-bindings: display: xlnx: Add ZynqMP DP subsystem bindings

The bindings describe the ZynqMP DP subsystem. They don't support the
interface with the programmable logic (FPGA) or audio yet.

Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>