media: v4l: fwnode: Add convenience function for parsing common external refs

Add v4l2_fwnode_parse_reference_sensor_common for parsing common
sensor properties that refer to adjacent devices such as flash or lens
driver chips.

As this is an association only, there's little a regular driver needs to
know about these devices as such.

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