selftests: mlxsw: Add Spectrum-2 mirror-to-gretap target scale test

Like in Spectrum, use the number of analyzers taken from the devlink
command.

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>