cpufreq: CPPC: Add per_cpu efficiency_class

In ACPI, describing power efficiency of CPUs can be done through the
following arm specific field:
ACPI 6.4, s5.2.12.14 'GIC CPU Interface (GICC) Structure',
'Processor Power Efficiency Class field':
  Describes the relative power efficiency of the associated pro-
  cessor. Lower efficiency class numbers are more efficient than
  higher ones (e.g. efficiency class 0 should be treated as more
  efficient than efficiency class 1). However, absolute values
  of this number have no meaning: 2 isn’t necessarily half as
  efficient as 1.

The efficiency_class field is stored in the GicC structure of the
ACPI MADT table and it's currently supported in Linux for arm64 only.
Thus, this new functionality is introduced for arm64 only.

To allow the cppc_cpufreq driver to know and preprocess the
efficiency_class values of all the CPUs, add a per_cpu efficiency_class
variable to store them.

At least 2 different efficiency classes must be present,
otherwise there is no use in creating an Energy Model.

The efficiency_class values are squeezed in [0:#efficiency_class-1]
while conserving the order. For instance, efficiency classes of:
  [111, 212, 250]
will be mapped to:
  [0 (was 111), 1 (was 212), 2 (was 250)].

Each policy being independently registered in the driver, populating
the per_cpu efficiency_class is done only once at the driver
initialization. This prevents from having each policy re-searching the
efficiency_class values of other CPUs. The EM will be registered in a
following patch.

The patch also exports acpi_cpu_get_madt_gicc() to fetch the GicC
structure of the ACPI MADT table for each CPU.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>