mlxsw: spectrum_acl: Extend Spectrum-2 region struct

In a similar fashion to Spectrum-1's region struct, Spectrum-2's struct
needs to store a pointer to the common region struct.

The pointer will be used in follow-up patches that implement rules
insertion and deletion.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>