dt: bindings: drm: dp: Add support of ZynqMP DP subsystem

In Xilinx ZynqMP, the DisplayPort IP core is integrated into
the DisplayPort subsystem. When the phandle to the DisplayPort subsystem
exsits in DP DT node, the ZynqMP DP subsystem is enabled by the driver,
and the DP driver uses the DP subsystem features. Add the corresponding
compatible string.

Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>