perf/arm-cmn: Support CMN S3 r2 isolation feature

The r2p0 release of CMN S3 finally adds a visible indication of child
node isolation status, so we can support configurations with the feature
enabled. It also brings some functional PMU changes such that certain
events won't quite work properly as-is, but that's a job for another day.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>