selftests: mlxsw: Add target for mirror-to-gretap test on spectrum

Add a wrapper around mlxsw/mirror_gre_scale.sh that parameterized number
of offloadable mirrors on Spectrum machines.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>