ACPI: CPPC: Add helper to get the highest performance value

Add support for getting the highest performance to the
generic CPPC driver. This enables downstream drivers
such as amd-pstate to discover and use these values.

Refer to Chapter 8.4.6.1.1.1. Highest Performance of ACPI
Specification 6.5 for details on continuous performance control
of CPPC (linked below).

Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Wyes Karny <wyes.karny@amd.com>
Reviewed-by: Perry Yuan <perry.yuan@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Meng Li <li.meng@amd.com>
Link: https://uefi.org/specs/ACPI/6.5/08_Processor_Configuration_and_Control.html?highlight=cppc#highest-performance


[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>