Like arm-cci, arm-ccn has the same issue where disabling preemption to
avoid races between registering the PMU device and the hotplug notifier
can lead to those operations taking mutexes in an invalid context. Fix
it the same way by disabling hotplug instead of preemption. Since we
only ever associate the PMU instance with a single CPU, we can also take
the opportunity to slightly simplify the hotplug handling to track just
that CPU number instead of a full cpumask.
Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>