perf pmu: Add helper function is_pmu_core to detect PMU CORE devices

perf pmu: Add helper function is_pmu_core to detect PMU CORE devices

On some platforms, PMU core devices sysfs name is not cpu.
Adding function is_pmu_core to detect PMU core devices using
core device specific hints in sysfs.

For arm64 platforms, all core devices have file "cpus" in sysfs.

Signed-off-by: Ganapatrao Kulkarni <[email protected]>
Tested-by: Shaokun Zhang <[email protected]>
Tested-by: Jin Yao <[email protected]>
Acked-by: Will Deacon <[email protected]>
Link: https://lkml.kernel.org/n/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>