nfp: abm: add initial active buffer management NIC skeleton

Add a very rudimentary active buffer management NIC support.
For now it's like a core NIC without SR-IOV support.  Next
commits will extend its functionality.

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>