platform/x86: intel_pmc_core: Fix file permission warnings

Symbolic permissions 'S_IRUGO' are not preferred. This patch changes the
debugfs files to use octal permissions '0644' or '0444' as needed by the
attribute.

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>