media: dt: bindings: Add a binding for flash LED devices associated to a sensor

Camera flash drivers (and LEDs) are separate from the sensor devices in
DT. In order to make an association between the two, provide the
association information to the software.

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