[media] media: entity: Add get_fwnode_pad entity operation

The optional operation can be used by entities to report how it maps its
fwnode endpoints to media pad numbers. This is useful for devices which
require advanced mappings of pads.

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