dt-bindings: video: Add for rockchip display subsytem

This add a display subsystem comprise the all display interface nodes.
Signed-off-by: Mark Yao <mark.yao@rock-chips.com>

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

+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>;
+};