drm/bridge: Add Analogix anx6345 support

The ANX6345 is an ultra-low power DisplayPower/eDP transmitter designed
for portable devices. This driver adds initial support for RGB to eDP
mode, without HPD and interrupts.

This is a configuration usually seen in eDP applications.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Torsten Duwe <duwe@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107135220.590D968BFE@verein.lst.de