media: staging/media: add a STAGING_MEDIA_DEPRECATED option

Add a kernel config option to build deprecated media drivers
that are scheduled for removal. Move stkwebcam to the deprecated
directory to make it clear that this driver is deprecated.

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