perf/arm-cci: Simplify CPU hotplug

Realistically, systems with multiple CCIs are unlikely to ever exist,
and since the driver only actually supports a single instance anyway
there's really no need to do the multi-instance hotplug state dance.

Take the opportunity to simplify the hotplug-related code all over,
addressing the context-migration TODO in the process for good measure.

Acked-by: Punit Agrawal <punit.agrawal@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>