net/mlx5: E-Switch, Support getting chain mapping

Currently, we write chain register mapping on miss from the the last
prio of a chain. It is used to restore the chain in software.

To support re-using the chain register mapping from global tables (such
as CT tuple table) misses, export the chain mapping.

Signed-off-by: Paul Blakey <paulb@mellanox.com>
Reviewed-by: Oz Shlomo <ozsh@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>