dt-bindings: video: Add for rockchip display subsytem

Rockchip DRM master device

================================

The Rockchip DRM master device is a virtual device needed to list all

vop devices or other display interface nodes that comprise the

graphics subsystem.

Required properties:

- compatible: Should be "rockchip,display-subsystem"

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

  of vop devices. vop definitions as defined in

  Documentation/devicetree/bindings/video/rockchip-vop.txt

example:

display-subsystem {

	compatible = "rockchip,display-subsystem";

	ports = <&vopl_out>, <&vopb_out>;

};