nfp: flower-ct: add tc_merge_tb

Add the table required to store the merge result of pre_ct and post_ct
flows. This is just the initial setup and teardown of the table,
the implementation will be 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>