[media] vivid: add CEC emulation

The vivid driver has been extended to provide CEC adapters for the HDMI
input and HDMI outputs in order to test CEC applications.

This CEC emulation is faithful to the CEC timings (i.e., it all at a
snail's pace).

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>