selftests: mlxsw: Add target for tc flower test on spectrum

Add a wrapper around mlxsw/tc_flower_scale.sh that parameterizes the
generic tc flower scale test template with Spectrum-specific target
values.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Yuval Mintz <yuvalm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>