V4L/DVB (12548): v4l2: video device: Add V4L2_CTRL_CLASS_FM_TX controls

This patch adds a new class of extended controls. This class
is intended to support FM Radio Modulators properties such as:
rds, audio limiters, audio compression, pilot tone generation,
tuning power levels and preemphasis properties.

Signed-off-by: Eduardo Valentin <eduardo.valentin@nokia.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>