[media] v4l2: Add documentation for the FM RX controls

Add appropriate documentation for all the newly added standard
controls.
Based on the patch by Manjunatha Halli [1]
[1] http://lists-archives.com/linux-kernel/27641303-media-update-docs-for-v4l2-fm-new-features.html



Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>