net: devlink: add port_init/fini() helpers to allow pre-register/post-unregister functions

Lifetime of some of the devlink objects, like regions, is currently
forced to be different for devlink instance and devlink port instance
(per-port regions). The reason is that for devlink ports, the internal
structures initialization happens only after devlink_port_register() is
called.

To resolve this inconsistency, introduce new set of helpers to allow
driver to initialize devlink pointer and region list before
devlink_register() is called. That allows port regions to be created
before devlink port registration and destroyed after devlink
port unregistration.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>