nfp: introduce nfp_port

Encapsulate port information into struct nfp_port.  nfp_port will
soon be extended to contain devlink_port information.  It also makes
it easier to reuse port-related code between vNICs and representors.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>