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

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-core.c, media/cec.h and cec-priv.h sources.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
[k.debski@samsung.com: Merged CEC Updates commit by Hans Verkuil]
[k.debski@samsung.com: Merged Update author commit by Hans Verkuil]
[k.debski@samsung.com: code cleanup and fixes]
[k.debski@samsung.com: add missing CEC commands to match spec]
[k.debski@samsung.com: add RC framework support]
[k.debski@samsung.com: move and edit documentation]
[k.debski@samsung.com: add vendor id reporting]
[k.debski@samsung.com: reorder of API structs and add reserved fields]
[k.debski@samsung.com: fix handling of events and fix 32/64bit timespec problem]
[k.debski@samsung.com: add sequence number handling]
[k.debski@samsung.com: add passthrough mode]
[k.debski@samsung.com: fix CEC defines, add missing CEC 2.0 commands]
Signed-off-by: Kamil Debski <kamil@wypas.org>

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