nfp: flower-ct: add nft_merge table

Add table and struct to save the result of the three-way merge
between pre_ct,post_ct, and nft flows. Merging code is to be
added in follow-up patches.

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>