media: vivid: add CEC pin monitoring emulation

Add support to emulate CEC pin monitoring. There are few hardware devices
that support this, so being able to emulate it here helps developing
software for this.

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