V4L/DVB (12554): FM TX: si4713: Add document file

This patch adds a document file for si4713 device driver.
It describes the driver interfaces and organization.

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>