dt-bindings: drm/bridge: analogix-anx78xx: add new variants

Add support for the analogix,anx7808, analogix,anx7812, and
analogix,anx7818 variants.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190815004854.19860-2-masneyb@onstation.org