[media] devices.txt: add video4linux device for Software Defined Radio

Add new video4linux device named /dev/swradio for Software Defined
Radio use. V4L device minor numbers are allocated dynamically
nowadays, but there is still configuration option for old fixed style.
Add note to mention that configuration option too.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>