[media] cec: fix Kconfig dependency problems

- Use IS_REACHABLE(RC_CORE) instead of IS_ENABLED: if cec is built-in and
  RC_CORE is a module, then CEC can't reach the RC symbols.
- Both cec and cec-edid should be bool and use the same build 'mode' as
  MEDIA_SUPPORT (just as is done for the media controller code).
- Add a note to staging that this should be changed once the cec framework
  is moved out of staging.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>