ANDROID: cpufreq: provide default frequency-invariance setter function

Frequency-invariant accounting support based on the ratio of current
frequency and maximum supported frequency is an optional feature an arch
can implement.

Since there are cpufreq drivers (e.g. cpufreq-dt) which can be build for
different arch's a default implementation of the frequency-invariance
setter function arch_set_freq_scale() is needed.

This default implementation is an empty weak function which will be
overwritten by a strong function in case the arch provides one.

The setter function passes the cpumask of related (to the frequency
change) cpus (online and offline cpus), the (new) current frequency and
the maximum supported frequency.

Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Change-Id: I912d5815ee29e1171c498e638d1a089c5a598add
Signed-off-by: Chris Redpath <chris.redpath@arm.com>