arm_pmu: ACPI support

Hi,

This series implements ACPI support in the ARM PMU code. It borrows some code from Jeremy's series [1], but takes a different approach to probing and association, using the usual hotplug state machine, minimising external changes required, and simplifying the relationship with the common arm_pmu code.

The first few patches are preparatory cleanup/refactoring, with the latter half of the series being specific to ACPI support.

The series is based on Will's perf/updates branch [2]. I've pushed the whole series out to the arm/perf/acpi branch [3] of my kernel.org repo.

Due to the innards of the hotplug callback framework, it's not entirely safe to register a PMU in a hotplug callback. Due to this, we can only associated hotplugged CPUs with a PMU if a matching CPU was around at probe time. A similar restriction already applies to DT systems. We may be able to relax this with some future work.

I've given this some testing on a Juno platform (using SPIs). To see that IRQs are correctly associated, I've tested with the following:

  $ taskset -c ${SOME_CPU_HERE} perf record \
    -e armv8_pmuv3_0/cpu_cycles/ \
    -e armv8_pmuv3_1/cpu_cycles/ \
    cat /proc/interrupts

I've also booted with nr_cpus temporarily capped (passing maxcpus=) to test the association logic. This has also been tested in a VM using PPIs; I do not have access to a host machine which itself uses PPIs.