ACPI / CPPC: add sysfs support to compute delivered performance

ACPI / CPPC: add sysfs support to compute delivered performance

The CPPC tables contain entries for per CPU feedback counters which
allows us to compute the delivered performance over a given interval
of time.

The math for delivered performance per the CPPCv5.0+ spec is:
  reference perf * delta(delivered perf ctr)/delta(ref perf ctr)

Maintaining deltas of the counters in the kernel is messy, as it
depends on when the reads are triggered. (e.g. via the cpufreq
->get() interface). Also the ->get() interace only returns one
value, so cant return raw values. So instead, leave it to userspace
to keep track of raw values and do its math for CPUs it cares about.

delivered and reference perf counters are exposed via the same
sysfs file to avoid the potential "skid", if these values are read
individually from userspace.

Signed-off-by: Prashanth Prakash <pprakash@codeaurora.org>
Signed-off-by: Ashwin Chaugule <ashwin.chaugule@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>