platform/x86: intel_telemetry: Add debugfs entry for S0ix residency

This adds a debugfs consumer for the exported kernel API
intel_pmc_read_s0ix_residency. This debugfs entry reads S0ix residency
directly from the PMC hardware counters.

TEST:
- echo freeze > /sys/power/state
- Wake the system, read the S0ix residency i.e.
  cat /sys/kernel/debug/telemetry/s0ix_residency_usec

Signed-off-by: Shanth Murthy <shanth.murthy@intel.com>
Signed-off-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>