mlxsw: spectrum: Add support for IPv6 traps

Before we can start using IPv6, we need to trap certain control packets
to the CPU. Among others, these include Neighbour Discovery, DHCP and
neighbour misses.

Signed-off-by: Arkadi Sharshevsky <arkadis@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>