Perf: arm64: disable irq for hotplug offline

Currently the hotplug notifier calls cpu_pmu_enable_percpu_irq
when a CPU comes online. The notifier doesn't have a corresponding
call to cpu_pmu_disable_percpu_irq when the CPU goes offline.
Change the code to follow what was done in 3.18 including
tracking whether the PMU is active and the IRQ number in use.

Change-Id: I1835fb543eb483713a30eb6c0ccd4e4a4b9908be
Signed-off-by: Patrick Fay <pfay@codeaurora.org>