cpufreq: powernv: Report cpu frequency throttling

The power and thermal safety of the system is taken care by an
On-Chip-Controller (OCC) which is real-time subsystem embedded within
the POWER8 processor. OCC continuously monitors the memory and core
temperature, the total system power, state of power supply and fan.

The cpu frequency can be throttled by OCC for the following reasons:
1)If a processor crosses its power and temperature limit then OCC will
  lower its Pmax to reduce the frequency and voltage.
2)If OCC crashes then the system is forced to Psafe frequency.
3)If OCC fails to recover then the kernel is not allowed to do any
  further frequency changes and the chip will remain in Psafe.

The user can see a drop in performance when frequency is throttled and
is unaware of throttling. So detect and report such a condition, so
the user can check the OCC status to reboot the system or check for
power supply or fan failures.

The current status of the core is read from Power Management Status
Register(PMSR) to check if any of the throttling condition is occurred
and the appropriate throttling message is reported.

Signed-off-by: Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>
Reviewed-by: Preeti U Murthy <preeti@linux.vnet.ibm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>