dt-bindings: display: renesas: lvds: Add renesas,companion property

Add a new optional renesas,companion property to point to the companion
LVDS encoder. This is used to support dual-link operation where the main
LVDS encoder splits even-numbered and odd-numbered pixels between the
two LVDS encoders.

The new property doesn't control the mode of operation, it only
describes the relationship between the master and companion LVDS
encoders.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Tested-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>

Documentation/devicetree/bindings/display/bridge/renesas,lvds.txt

@@ -46,14 +46,24 @@ OF graph bindings specified in Documentation/devicetree/bindings/graph.txt.
 
 Each port shall have a single endpoint.
 
+Optional properties:
+
+- renesas,companion : phandle to the companion LVDS encoder. This property is
+  mandatory for the first LVDS encoder on D3 and E3 SoCs, and shall point to
+  the second encoder to be used as a companion in dual-link mode. It shall not
+  be set for any other LVDS encoder.
+
 
 Example:
 
 	lvds0: lvds@feb90000 {
-		compatible = "renesas,r8a7790-lvds";
-		reg = <0 0xfeb90000 0 0x1c>;
-		clocks = <&cpg CPG_MOD 726>;
-		resets = <&cpg 726>;
+		compatible = "renesas,r8a77990-lvds";
+		reg = <0 0xfeb90000 0 0x20>;
+		clocks = <&cpg CPG_MOD 727>;
+		power-domains = <&sysc R8A77990_PD_ALWAYS_ON>;
+		resets = <&cpg 727>;
+
+		renesas,companion = <&lvds1>;
 
 		ports {
 			#address-cells = <1>;