arm64: dts: zynqmp: Add DisplayPort subsystem

Add a DT node for the DisplayPort subsystem, a hard IP present in the
Zynq Ultrascale+ MPSoC.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/4d978aef852cacdfb35aa8e50d648a787e73b90c.1611232558.git.michal.simek@xilinx.com