nfp: flower-ct: add nfp_fl_ct_flow_entries

This commit starts adding the structures and lists that will
be used in follow up commits to enable offloading of conntrack.
Some stub functions are also introduced as placeholders by
this commit.

Signed-off-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Yinjun Zhang <yinjun.zhang@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>