selftests: mlxsw: Add a resource scale test for Spectrum-2

Add resource_scale test suitable for Spectrum-2.

Invoke the mirror_gre test and check that the advertised scale numbers
are indeed supported.

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