tools/power turbostat: Honor --show CPU, even when even when num_cpus=1

Honor --show CPU and --show Core when "topo.num_cpus == 1".
Previously turbostat assumed that on a 1-CPU system, these
columns should never appear.

Honoring these flags makes it easier for several programs
that parse turbostat output.

Signed-off-by: Patryk Wlazlyn <patryk.wlazlyn@linux.intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>