arm_pmu: add armpmu_alloc_atomic()

In ACPI systems, we don't know the makeup of CPUs until we hotplug them
on, and thus have to allocate the PMU datastructures at hotplug time.
Thus, we must use GFP_ATOMIC allocations.

Let's add an armpmu_alloc_atomic() that we can use in this case.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>