nfp: abm: add support for more threshold actions

Original FW only allowed us to perform ECN marking.  Newer releases
also support plain old drop.  Add the ability to configure drop
policy.  This is particularly useful in combination with GRED,
because different bands can have different ECN marking setting.

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>