nfp: abm: add GRED offload

Add support for GRED offload.  It behaves much like RED, but
can apply different parameters to different bands.  GRED operates
pretty much exactly like our HW/FW with a single FIFO and different
RED state instances.

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>