mlxsw: reg: Add the Router Interface Group Version 2 register

The RIGR-V2 register is used to add, remove and query egress interface list
of a multicast forwarding entry and it will be used by the multicast
router offloading logic.

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