dt-bindings: Add ANX6345 DP/eDP transmitter binding

The anx6345 is an ultra-low power DisplayPort/eDP transmitter designed
for portable devices.

Add a binding document for it.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Torsten Duwe <duwe@suse.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>