acpi: add a driver for ACPI CPPC

[cognet]

Summary

Add a new driver, acpi_cppc, for ACPI CPPC (Collaborative Processor
Performance Control). When ACPI provides _CPC, this makes it possible
to control the CPU frequency via cpufreq.
Oonly System Memory registers are supported. PCC is not, as I have no hardware
to test it on, and Functional Fixed Hardware is already supported by other
drivers.
There is nothing arm64-specific in there, however for now it is only built
for arm64, as I haven't tested it anywhere else, and this is the only
platform I expect it to be used for now. The day it is needed somewhere else,
we can just make it MI.
This allows me to control the CPUs frequency on my Radxa Orion O6.