platform/x86: intel_pmc_core: Refactor debugfs entries

When on a platform if we can't show MPHY and PLL status, don't even bother
to create a debugfs entry as it will fail anyway. In fact unless OEM builds
a special BIOS for test, it will fail on every production system.

This will help to add future platform support where we can't support these
entries.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>