Contrary to the previous implementation, it now calls the cpufreq driver,
and reads out the _actual_ current frequency, and not the frequency the
CPUfreq core _thinks_ the CPU is running at. Most cpufreq drivers do provide
such a "hw get" function (only ACPI-io can definitely not be supported,
I'm not sure about sh, sparc64 and powermac) anyway, and it is useful for
other issues.