intel_pmc_core: Convert to DEFINE_DEBUGFS_ATTRIBUTE

Refactor the code to use the recently introduced
DEFINE_DEBUGFS_ATTRIBUTE() macro to eliminate boilerplate code.
Make the absence of DEBUG_FS a non-fatal error.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-and-tested-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@intel.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>