dt-bindings: ili9881c: add missing panel-common inheritance

The properties below refer to items in panel-common.yaml, which means
that needs to be referenced in the schema.

Signed-off-by: John Keeping <[e-mail]>
Signed-off-by: Sam Ravnborg <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

Documentation/devicetree/bindings/display/panel/ilitek,ili9881c.yaml

···
 maintainers:
   - Maxime Ripard <[e-mail]>
 
+allOf:
+  - $ref: panel-common.yaml#
+
 properties:
   compatible:
     items: