nfp: add framework to support ipsec offloading

A new metadata type and config structure are introduced to
interact with firmware to support ipsec offloading. This
feature relies on specific firmware that supports ipsec
encrypt/decrypt by advertising related capability bit.

The xfrm callbacks which interact with upper layer are
implemented in the following patch.

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>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>