nfp: abm: add cls_u32 offload for simple band classification

Use offload of very simple u32 filters to direct packets to GRED
bands based on the DSCP marking.  No u32 hashing is supported,
just plain simple filters matching on ToS or Priority with
appropriate mask device can support.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: John Hurley <john.hurley@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>