V4L/DVB (11281): bttv: move saa6588 config to the helper chip config

saa6588 can also be used by other drivers than just bttv. Move it to a
new RDS decoders category and add it as helper chip to bttv.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>