devlink: Prepare code to fill multiple port function attributes

Prepare code to fill zero or more port function optional attributes.
Subsequent patch makes use of this to fill more port function
attributes.

Signed-off-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Vu Pham <vuhuong@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>