perf test pmu: Warn don't fail for legacy mixed case event names

PowerPC has mixed case events matching legacy hardware cache
events. Warn but don't fail in this case. Event parsing will still
work in this case by matching the legacy case.

Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Kajol Jain <kjain@linux.ibm.com>
Cc: James Clark <james.clark@arm.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240612124027.2712643-1-irogers@google.com