[media] cec: add HDMI CEC framework (api)

The added HDMI CEC framework provides a generic kernel interface for
HDMI CEC devices.

Note that the CEC framework is added to staging/media and that the
cec.h and cec-funcs.h headers are not exported yet. While the kABI
is mature, I would prefer to allow the uABI some more time before
it is mainlined in case it needs more tweaks.

This adds the cec-api.c source that deals with the public CEC API
and the Kconfig/Makefile plumbing.

The MAINTAINERS file is also updated.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
[k.debski@samsung.com: code cleanup and fixes]
Signed-off-by: Kamil Debski <kamil@wypas.org>

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>