devlink: push object register/unregister notifications into separate helpers

In preparations of leftover.c split to individual files, avoid need to
have object structures exposed in devl_internal.h and allow to have them
maintained in object files.

The register/unregister notifications need to know the structures
to iterate lists. To avoid the need, introduce per-object
register/unregister notification helpers and use them.
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20230828061657.300667-2-jiri@resnulli.usSigned-off-by: Jakub Kicinski <kuba@kernel.org>