perf vendor events arm64: Fix for AmpereOne metrics

This patch addresses review comments that were given for
705ed54 ("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/[email protected]/
[2] https://lore.kernel.org/linux-perf-users/[email protected]/

Fixes: 705ed54 ("perf vendor events arm64: Add AmpereOne metrics")
Signed-off-by: Ilkka Koskinen <[email protected]>
Reviewed-by: Ian Rogers <[email protected]>
Cc: James Clark <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Leo Yan <[email protected]>
Cc: Mike Leach <[email protected]>
Cc: Dave Kleikamp <[email protected]>
Cc: John Garry <[email protected]>
Cc: D Scott Phillips <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Namhyung Kim <[email protected]>