drm/bridge: analogix-anx78xx: add new variants

Add support for the 7808 variant. While we're here, the of match table
was missing support for the 7812 and 7818 variants, so add them as well.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190815004854.19860-3-masneyb@onstation.org