drm: Add adv7511 encoder driver

This patch adds a driver for the Analog Devices adv7511. The adv7511 is
a standalone HDMI transmitter chip. It features a HDMI output interface
on one end and video and audio input interfaces on the other.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>