V4L/DVB (4513): Split audio decoders from bttv

Several audio decoders were just bound as if they were part of
bttv driver. In fact, they are generic enough to be used by other
drivers as well, since they use I2C event bus to communicate.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>