[media] v4l2: Add standard controls for FM receivers

This commit introduces new class of standard controls
V4L2_CTRL_CLASS_FM_RX. This class is intended to all controls
pertaining to FM receiver chips. Also, two controls belonging to said
class are added as a part of this commit: V4L2_CID_TUNE_DEEMPHASIS and
V4L2_CID_RDS_RECEPTION.
This patch is based on the code found in the patch by Manjunatha Halli [1]
[1] http://lists-archives.com/linux-kernel/27641307-new-control-class-and-features-for-fm-rx.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>