mlxsw: reg: Add register settings for IPv6 multicast routing

Add new fields for the rmft register necessary for setting the IPv6
multicast FIB table. Add a matching wrapper function for filling
the register in the IPv6 scenario.

Signed-off-by: Yuval Mintz <yuvalm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>