perf/arm-cmn: Fix PMU instance naming

perf/arm-cmn: Fix PMU instance naming

Although it's neat to avoid the suffix for the typical case of a
single PMU, it means systems with multiple CMN instances end up with
inconsistent naming. I think it also breaks perf tool's "uncore alias"
logic if the common instance prefix is also the full name of one.

Avoid any surprises by not trying to be clever and simply numbering
every instance, even when it might technically prove redundant.

Fixes: 0ba64770a2f2 ("perf: Add Arm CMN-600 PMU driver")
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/649a2281233f193d59240b13ed91b57337c77b32.1611839564.git.robin.murphy@arm.com
Signed-off-by: Will Deacon <will@kernel.org>