V4L/DVB (12551): FM TX: si4713: Add files to add radio interface for si4713

This patch adds files which creates the radio interface
for si4713 FM transmitter (modulator) devices.

In order to do the real access to device registers, this
driver uses the v4l2 subdev interface exported by si4713 i2c driver.

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>