mlxsw: spectrum: Add multicast router trap for PIMv6

Add a new trap for PIMv6 packets. As PIM already has a designated trap
group [ & rate limiter], simply use the same for PIMv6 as well.

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>