mlxsw: reg: Adding SMID register

Adding back SMID register definition and packing. For each MC group a new
SMID entry will be generated.

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