media: v4l: fwnode: Add a helper function to obtain device / integer references

v4l2_fwnode_reference_parse_int_prop() will find an fwnode such that under
the device's own fwnode, it will follow child fwnodes with the given
property-value pair and return the resulting fwnode.

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