dt-bindings: display: renesas: du: Document optional reset properties

Document the optional properties for describing module resets, to
support resetting display channels on R-Car Gen2 and Gen3.

Signed-off-by: Geert Uytterhoeven <[e-mail]>
Acked-by: Laurent Pinchart <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Reviewed-by: Kieran Bingham <[e-mail]>
Signed-off-by: Laurent Pinchart <[e-mail]>

Documentation/devicetree/bindings/display/renesas,du.txt

···
     VSP instance that serves the DU channel, and the channel index identifies
     the LIF instance in that VSP.
 
+Optional properties:
+  - resets: A list of phandle + reset-specifier pairs, one for each entry in
+    the reset-names property.
+  - reset-names: Names of the resets. This property is model-dependent.
+    - All but R8A7779 use one reset for a group of one or more successive
+      channels. The resets must be named "du.x" with "x" being the numerical
+      index of the lowest channel in the group.
+
 Required nodes:
 
 The connections to the DU output video ports are modeled using the OF graph
···
 			 <&cpg CPG_MOD 722>,
 			 <&cpg CPG_MOD 721>;
 		clock-names = "du.0", "du.1", "du.2", "du.3";
+		resets = <&cpg 724>, <&cpg 722>;
+		reset-names = "du.0", "du.2";
 		renesas,cmms = <&cmm0>, <&cmm1>, <&cmm2>, <&cmm3>;
 		renesas,vsps = <&vspd0 0>, <&vspd1 0>, <&vspd2 0>, <&vspd0 1>;