platform/x86: intel_pmc_core: Make pmc_core_substate_res_show() generic

Currently pmc_core_substate_res_show() uses array of char pointers
i.e., lpm_modes for Tiger Lake directly to iterate through and to get
the number of low power modes which is hardcoded and cannot be re-used
for future platforms that support sub-states. To maintain readability,
make pmc_core_substate_res_show() generic, so that it can re-used for
future platforms.

Cc: Chen Zhou <chenzhou10@huawei.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: David E. 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>