Some Chromebook BIOS' do not export an ACPI LPIT, which is how
Linux finds the residency counter for CPU and SYSTEM low power states,
that is exports in /sys/devices/system/cpu/cpuidle/*residency_us

When these sysfs attributes are missing, check the debugfs attrubte
from the pmc_core driver, which accesses the same counter value.
Signed-off-by: Len Brown <len.brown@intel.com>