[media] vivid: add support for software defined radio

This adds support for an SDR capture device. It generates simple
sine/cosine waves. The code for that has been contributed by
Antti.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>