selftests/powerpc/pmu: Update comment with details to understand auxv_generic_compat_...

selftests/powerpc/pmu: Update comment with details to understand auxv_generic_compat_pmu() utility function

auxv_generic_compat_pmu() utility function is to detect whether the
system is having generic compat PMU. The check is based on base platform
value from /proc/self/auxv. Update the comment with details on how auxv
is used to detect the platform.

Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250113075858.45137-5-atrajeev@linux.vnet.ibm.com