nfp: abm: spawn port netdevs

To configure buffering points we need full set of netdevs:

                              ASIC

 user netdev  -- | -- PCIe port   MAC port -- | --

Configuring egrees qdiscs on user netdev configures standard
Linux TC software qdiscs, configuring PCIe port qdiscs will
provide a way of setting ASIC queuing parameters for PCIe block.
MAC port netdev egress qdiscs correspond to ASIC MAC Traffic
Manager block.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>