dt-bindings: display: xlnx: zynqmp-dpsub: Add OF graph ports

The DPSUB doesn't live in isolation, but is connected to the
programmable logic for live inputs and outputs, and also has a
DisplayPort output. Model all those using OF graph.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rob Herring <robh@kernel.org>