media: saa7146: deprecate hexium_gemini/orion, mxb and ttpci

Deprecate the hexium_gemini, hexium_orion, mxb and ttpci saa7146-based
drivers: these drivers do not use the vb2 framework for video streaming,
instead it uses the old videobuf framework.

We want to get rid of these old drivers, so deprecated these for future
removal.

[hverkuil: update MAINTAINERS file]

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>