nfp: flower: add GRE encap action support

Add new GRE encapsulation support, which allows offload of filters
using tunnel_key set action in combination with actions that egress
to GRE type ports.

Signed-off-by: Pieter Jansen van Vuuren <pieter.jansenvanvuuren@netronome.com>
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>