mlxsw: spectrum: Rename ARP trap group

The ARP trap group will be used for IPv6 ND traps in the next patch, so
rename it to "NEIGH_DISCOVERY" which is more appropriate.

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