drm/bridge/analogix/anx78xx: Add missing definition

Analogix ANX78XX driver is missing definitions for anx7816.
It uses the same I2C register set as anx7818.

Signed-off-by: Alicja Michalska <ahplka19@gmail.com>
Reviewed-by: Robert Foss <rfoss@kernel.org>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/ZPvagaXnQ/TlNEkJ@tora