perf/arm-cmn: Add sysfs identifier

Expose a sysfs identifier encapsulating the CMN part number and revision
so that jevents can narrow down a fundamental set of possible events for
calculating metrics. Configuration-dependent aspects - such as whether a
given node type is present, and/or a given node ID is valid - are still
not covered, and in general it's hard to see how userspace could handle
them, so we won't be removing any data or validation logic from the
driver any time soon, but at least it's a step in a useful direction.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-and-tested-by: Ilkka Koskinen <ilkka@os.amperecomputing.com>
Tested-by: Jing Zhang <renyu.zj@linux.alibaba.com>
Link: https://lore.kernel.org/r/b8a14c14fcdf028939ebf57849863e8ae01743de.1686588640.git.robin.murphy@arm.com


Signed-off-by: Will Deacon <will@kernel.org>