nfp: support port splitting via devlink

Add support for configuring port split with devlink.  Add devlink
callbacks to validate requested config and call NSP helpers.
Getting the right nfp_port structure can be done with simple iteration.

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>