dt-bindings: display/panel: Add missing unit names

Some examples were missing the unit names triggering

 Warning (unit_address_vs_reg): .../panel: node has a reg or ranges property, but no unit name

warnings when used verbatim in DTs and running dtc with W=1.
Signed-off-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/901e836ea06889a9d91a799102b2a6b836d93dcd.1553529797.git.agx@sigxcpu.org

Documentation/devicetree/bindings/display/panel/innolux,p079zca.txt

@@ -12,7 +12,7 @@ Optional properties:
 Example:
 
 	&mipi_dsi {
-		panel {
+		panel@0 {
 			compatible = "innolux,p079zca";
 			reg = <0>;
 			power-supply = <...>;

Documentation/devicetree/bindings/display/panel/innolux,p097pfg.txt

@@ -13,7 +13,7 @@ Optional properties:
 Example:
 
 	&mipi_dsi {
-		panel {
+		panel@0 {
 			compatible = "innolux,p079zca";
 			reg = <0>;
 			avdd-supply = <...>;

Documentation/devicetree/bindings/display/panel/kingdisplay,kd097d04.txt

@@ -12,7 +12,7 @@ Optional properties:
 Example:
 
 	&mipi_dsi {
-		panel {
+		panel@0 {
 			compatible = "kingdisplay,kd097d04";
 			reg = <0>;
 			power-supply = <...>;