[media] v4l: add device type for Software Defined Radio

Add new V4L device type VFL_TYPE_SDR for Software Defined Radio.
It is registered as /dev/swradio0 (/dev/sdr0 was already reserved).

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