drm: adv7511/33: add HDMI CEC support

Add support for HDMI CEC to the drm adv7511/adv7533 drivers.

The CEC registers that we need to use are identical for both drivers,
but they appear at different offsets in the register map.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20171007104658.14528-3-hverkuil@xs4all.nl