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 <john@metanate.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211020153432.383845-2-john@metanate.com