V4L/DVB (12552): FM TX: si4713: Add files to handle si4713 i2c device

This patch adds files to control si4713 devices.
Internal functions to control device properties
and initialization procedures are into these files.
Also, a v4l2 subdev interface is also exported.
This way other drivers can use this as v4l2 i2c subdevice.

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>