nfp: implement xfrm callbacks and expose ipsec offload feature to upper layer

Xfrm callbacks are implemented to offload SA info into firmware
by mailbox. It supports 16K SA info in total.

Expose ipsec offload feature to upper layer, this feature will
signal the availability of the offload.

Based on initial work of Norm Bagley <norman.bagley@netronome.com>.

Signed-off-by: Huanhuan Wang <huanhuan.wang@corigine.com>
Reviewed-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Acked-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>