cpuidle: powernv: Populate cpuidle state details by querying the device-tree

We hard code the metrics relevant for cpuidle states in the kernel today.
Instead pick them up from the device tree so that they remain relevant
and updated for the system that the kernel is running on.

Signed-off-by: Preeti U. Murthy <preeti@linux.vnet.ibm.com>
Signed-off-by: Shreyas B. Prabhu <shreyas@linux.vnet.ibm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>