powerpc: Add support for new hcall H_BEST_ENERGY

Create sysfs interface to export data from H_BEST_ENERGY hcall
that can be used by administrative tools on supported pseries
platforms for energy management	optimizations.

sys/device/system/cpu/pseries_(de)activate_hint_list and
sys/device/system/cpu/cpuN/pseries_(de)activate_hint will provide
hints for activation and deactivation of cpus respectively.

These hints are abstract number given by the hypervisor based
on the extended knowledge the hypervisor has regarding the
system topology and resource mappings.

The activate and the deactivate sysfs entry is for the two
distinct operations that we could do for energy savings.  When
we have more capacity than required, we could deactivate few
core to save energy.  The choice of the core to deactivate
will be based on /sys/devices/system/cpu/deactivate_hint_list.
The comma separated list of cpus (cores) will be the preferred
choice.  If we have to activate some of the deactivated cores,
then /sys/devices/system/cpu/activate_hint_list will be used.

The per-cpu file
/sys/device/system/cpu/cpuN/pseries_(de)activate_hint further
provide more fine grain information by exporting the value of
the hint itself.

Added new driver module
	arch/powerpc/platforms/pseries/pseries_energy.c
under new config option CONFIG_PSERIES_ENERGY

Signed-off-by: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>