perf vendor power9 nest metrics: Correct unit from MB to MiB

perf vendor power9 nest metrics: Correct unit from MB to MiB

6.1e-5 is very close to 1/16384, where 16384 is 2^14, i.e. a power of
2. When units are in powers of 2 the IEC unit is MiB (mebibytes)
rather than MB (megabytes) where the values are powers of 10.

This patch corrects the unit for uniformity and because such units may
be pattern matched against.

Reviewed-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Cc: Kajol Jain <kjain@linux.ibm.com>
Link: https://lore.kernel.org/r/20251031162637.1456191-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>