tools/testing/selftests/powerpc/pmu: Update comment description to mention ISA...

tools/testing/selftests/powerpc/pmu: Update comment description to mention ISA v3.1 for power10 and above

Updated the comments in the pmu selftests to include
power11/ISA v3.1 where ever required.

Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250113075858.45137-3-atrajeev@linux.vnet.ibm.com