devlink: introduce port ops placeholder

In devlink, some of the objects have separate ops registered alongside
with the object itself. Port however have ops in devlink_ops structure.
For drivers what register multiple kinds of ports with different ops
this is not convenient. Introduce devlink_port_ops and a set
of functions that allow drivers to pass ops pointer during
port registration.

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