V4L/DVB (8635): v4l: add AC-3 audio support to the MPEG Encoding API

Some models of the saa6752hs support AC-3. Extend the API with the
necessary controls for AC-3.

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