cpufreq / CPPC: Add cpuinfo_cur_freq support for CPPC

Per Section 8.4.7.1.3 of ACPI 6.2, the platform provides performance
feedback via set of performance counters. To determine the actual
performance level delivered over time, OSPM may read a set of
performance counters from the Reference Performance Counter Register
and the Delivered Performance Counter Register.

OSPM calculates the delivered performance over a given time period by
taking a beginning and ending snapshot of both the reference and
delivered performance counters, and calculating:

delivered_perf = reference_perf X (delta of delivered_perf counter / delta of reference_perf counter).

Implement the above and hook this up to the cpufreq->get method.

Signed-off-by: George Cherian <george.cherian@cavium.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Prashanth Prakash <pprakash@codeaurora.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>