drm/bridge: Add Analogix anx78xx support

Although there are other chips from the same family that can reuse this
driver, at the moment we only tested ANX7814 chip.

The ANX7814 is an ultra-low power Full-HD (1080p60) SlimPort transmitter
designed for portable devices. This driver adds initial support for HDMI
to DP pass-through mode.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Tested-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-by: Nicolas Boichat <drinkcat@chromium.org>
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Daniel Kurtz <djkurtz@chromium.org>
Cc: Nicolas Boichat <drinkcat@chromium.org>
Cc: Thierry Reding <treding@nvidia.com>
[treding@nvidia.com: coding style, propagate regulator_get() errors]
Signed-off-by: Thierry Reding <treding@nvidia.com>