[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>

Documentation/devices.txt

@@ -1491,10 +1491,17 @@ Your cooperation is appreciated.
 		 64 = /dev/radio0	Radio device
 		    ...
 		127 = /dev/radio63	Radio device
+		128 = /dev/swradio0	Software Defined Radio device
+		    ...
+		191 = /dev/swradio63	Software Defined Radio device
 		224 = /dev/vbi0		Vertical blank interrupt
 		    ...
 		255 = /dev/vbi31	Vertical blank interrupt
 
+		Minor numbers are allocated dynamically unless
+		CONFIG_VIDEO_FIXED_MINOR_RANGES (default n)
+		configuration option is set.
+
  81 block	I2O hard disk
 		  0 = /dev/i2o/hdq	17th I2O hard disk, whole disk
 		 16 = /dev/i2o/hdr	18th I2O hard disk, whole disk