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 <[e-mail]>
Signed-off-by: Madhavan Srinivasan <[e-mail]>
Link: https://patch.msgid.link/[e-mail]

tools/testing/selftests/powerpc/pmu/sampling_tests/misc.c

···
  * Utility function to check for generic compat PMU
  * by comparing base_platform value from auxv and real
  * PVR value.
+ * auxv_base_platform() func gives information of "base platform"
+ * corresponding to PVR value. Incase, if the distro doesn't
+ * support platform PVR (missing cputable support), base platform
+ * in auxv will have a default value other than the real PVR's.
+ * In this case, ISAv3 PMU (generic compat PMU) will be registered
+ * in the system. auxv_generic_compat_pmu() makes use of the base
+ * platform value from auxv to do this check.
  */
 static bool auxv_generic_compat_pmu(void)
 {