media: dt: bindings: Add lens-focus binding for image sensors

The lens-focus property contains a phandle to the lens voice coil driver
that is associated to the sensor; typically both are contained in the same
camera module.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Cc: devicetree@vger.kernel.org
Acked-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>