[media] si4713: add DT binding documentation

This patch adds the DT bindings documentation for Silicon Labs Si4713 FM
radio transmitter.

Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>