mlxsw: spectrum_router: Add support for route append

Handle append for gateway based routes. Dev-only multipath routes will
be handled by a follow on patch.

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