perf vendor events arm64: Fix for AmpereOne metrics

This patch addresses review comments that were given for
705ed549 ("perf vendor events arm64: Add AmpereOne metrics")
but didn't make it to the original patch [1][2]

Changes include: A fix for backend_memory formula, use of standard metrics
when possible, using #slots, renaming metrics to avoid spaces in the names,
and cleanup.

[1] https://lore.kernel.org/linux-perf-users/e9bdacb-a231-36af-6a2e-6918ee7effa@os.amperecomputing.com/
[2] https://lore.kernel.org/linux-perf-users/20230826192352.3043220-1-ilkka@os.amperecomputing.com/

Fixes: 705ed549 ("perf vendor events arm64: Add AmpereOne metrics")
Signed-off-by: Ilkka Koskinen <ilkka@os.amperecomputing.com>
Reviewed-by: Ian Rogers <irogers@google.com>
Cc: James Clark <james.clark@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Mike Leach <mike.leach@linaro.org>
Cc: Dave Kleikamp <dave.kleikamp@oracle.com>
Cc: John Garry <john.g.garry@oracle.com>
Cc: D Scott Phillips <scott@os.amperecomputing.com>
Cc: linux-arm-kernel@lists.infradead.org
Link: https://lore.kernel.org/r/20230920061839.2437413-1-ilkka@os.amperecomputing.comSigned-off-by: Namhyung Kim <namhyung@kernel.org>