[media] staging/media: add missing TODO files

Add TODO files for mx2/mx3/omap1 to explain the status of these drivers
and what needs to be done in order to keep them from being removed soon.

Also a small fix for the mx2/Kconfig that mistakingly mentioned a vb2
conversion. That's not needed for that driver.

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