x86/platform/intel/pmc_atom: Export accessors to PMC registers

x86/platform/intel/pmc_atom: Export accessors to PMC registers

Export the pmc_atom_read() and pmc_atom_write() accessors to the PMC
registers. On early initcall stages the functions will return
-ENODEV, and caller has to wait when it will be available.

Additionally make absence of debugfs a non-fatal error.

The patch will be useful for the upcoming fixes regarding to the
LPSS block found on Intel BayTrail-T and Braswell.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Aubrey Li <aubrey.li@linux.intel.com>
Cc: Kumar P Mahesh <mahesh.kumar.p@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rafael J . Wysocki <rafael.j.wysocki@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1436192944-56496-2-git-send-email-andriy.shevchenko@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>