dt-bindings: display: anx7814: Add definition for anx7816

As requested by Robert Foss <rfoss@kernel.org>, this patch adds
definition for anx7816. It supplements the patch submitted to dri-devel.

Signed-off-by: Alicja Michalska <ahplka19@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/ZQjFabKW7QvrvsnG@tora