mlxsw: spectrum_router: Process IP6MR fib notification

Following previous patches driver is ready to handle notifications
arriving from ip6mr - start processing those when they arrive following
the same manner ipmr currently goes through.

This should enable driver to start offloading ipv6 multicast routes.

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>