Device-Tree bindings for Phytium DRM master device

Phytium DRM master device is a virtual device needed to list all
display controller or other display interface nodes that comprise the
graphics subsystem.

Required properties:
- compatible: value should be "phytium,display-subsystem".

- ports: Should contain a list of phandles pointing to display interface ports
  of display controller devices

Example 1:

display-subsystem: display-subsystem-dcFTD330 {
	compatible = "phytium,display-subsystem";
	ports = <&dc_out_dpi0 &dc_out_dpi1>;
};

Example 2:

display_subsystem: display-subsystem-dc9400 {
	compatible = "phytium,display-subsystem";
	ports = <&dc_out_0 &dc_out_1 &dc_out_2 &dc_out_3 &dc_out_4>;
};