[media] btcx-risc: move to bt8xx

The btcx-risc module is no longer used by other drivers except for bttv.
So move it from common to bt8xx and make it part of the bttv driver instead
of as a separate module.

This module should never have been a common module since most of the code
has always been bttv specific.

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