ARM: perf: register cpu_notifier at driver init

The current practice of registering the cpu hotplug notifier at PMU
registration time won't be safe with multiple PMUs, as we'll repeatedly
attempt to register the notifier. This has the unfortunate effect of
silently corrupting the notifier list, leading to boot stalling.

Instead, register the notifier at init time. Its sanity checks will
prevent anything bad from happening if the notifier is called before we
have any PMUs registered.

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