mlxsw: spectrum_acl: Add support for Spectrum-2 block encoding

Encode each flexible key block in the general block scheme according its
block index.

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