dt-bindings: drm/bridge: ps8640: Add aux-bus child

dp-aux-bus.yaml says we can list an eDP panel as a child of
an eDP controller node to represent the fact that the panel
is connected to the controller's DP AUX bus.

Let's add it to the ps8640 bindings.
Signed-off-by: Philip Chen <philipchen@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210929173343.v2.2.I6050ba184b24d887e92692a72ee3054d643d6091@changeid

Documentation/devicetree/bindings/display/bridge/ps8640.yaml

@@ -40,6 +40,9 @@ properties:
   vdd33-supply:
     description: Regulator for 3.3V digital core power.
 
+  aux-bus:
+    $ref: /schemas/display/dp-aux-bus.yaml#
+
   ports:
     $ref: /schemas/graph.yaml#/properties/ports
 
@@ -98,7 +101,21 @@ examples:
                     reg = <1>;
                     ps8640_out: endpoint {
                         remote-endpoint = <&panel_in>;
-                   };
+                    };
+                };
+            };
+
+            aux-bus {
+                panel {
+                    compatible = "boe,nv133fhm-n62";
+                    power-supply = <&pp3300_dx_edp>;
+                    backlight = <&backlight>;
+
+                    port {
+                        panel_in: endpoint {
+                            remote-endpoint = <&ps8640_out>;
+                        };
+                    };
                 };
             };
         };