media: v4l: fwnode: Add a helper function for parsing generic references

Add function v4l2_fwnode_reference_parse() for parsing them as async
sub-devices. This can be done on e.g. flash or lens async sub-devices that
are not part of but are associated with a sensor.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>