selftests: mlxsw: Add router scale test for Spectrum-2

Same as for Spectrum-1, test the ability to add the maximum number of
routes possible to the switch.

Invoke the test from the 'resource_scale' wrapper script.

Signed-off-by: Danielle Ratson <danieller@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>