[media] radio-si470x doc: add info about v4l2-ctl and sox+alsa

Patch adds information about using v4l2-ctl utility to tune the si470x
radio and example how to use sox+alsa to redirect sound from radio sound
device to another sound device.

Signed-off-by: Alexey Klimov <klimov.linux@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>