platform/x86: intel_pmc_core: Add debugfs entry to access sub-state residencies

Prior to Tiger Lake, the platforms that support pmc_core have no
sub-states of S0ix. Tiger Lake has 8 sub-states/low power modes of S0ix
ranging from S0i2.0-S0i2.2 and S0i3.0-S0i3.4, simply represented as
S0ix.y.

Create a debugfs entry to access residency of each sub-state.

Cc: Srinivas Pandruvada <srinivas.pandruvada@intel.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: David Box <david.e.box@intel.com>
Signed-off-by: David Box <david.e.box@intel.com>
Signed-off-by: Gayatri Kammela <gayatri.kammela@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>