media: dt-bindings: Add binding for si470x radio

Add device tree bindings for si470x family radio receiver driver.

Signed-off-by: Pawe? Chmiel <pawel.mikolaj.chmiel@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>