devicetree: Add ANX7814 SlimPort transmitter binding

The ANX7814 is an ultra-low power Full-HD (1080p60) SlimPort transmitter
designed for portable devices.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>