mlxsw: spectrum_router: Add neighbour offload indication

In a similar fashion to routes and FDB entries, the neighbour table is
reflected to the device.

Set an offload indication on the neighbour in case it was programmed to
the device.

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