cpufreq: report whether cpufreq supports Frequency Invariance (FI)

Now that the update of the FI scale factor is done in cpufreq core for
selected functions - target(), target_index() and fast_switch(),
we can provide feedback to the task scheduler and architecture code
on whether cpufreq supports FI.

For this purpose provide an external function to expose whether the
cpufreq drivers support FI, by using a static key.

The logic behind the enablement of cpufreq-based invariance is as
follows:
 - cpufreq-based invariance is disabled by default
 - cpufreq-based invariance is enabled if any of the callbacks
   above is implemented while the unsupported setpolicy() is not

The cpufreq_supports_freq_invariance() function only returns whether
cpufreq is instrumented with the arch_set_freq_scale() calls that
result in support for frequency invariance. Due to the lack of knowledge
on whether the implementation of arch_set_freq_scale() actually results
in the setting of a scale factor based on cpufreq information, it is up
to the architecture code to ensure the setting and provision of the
scale factor to the scheduler.

Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

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