POWERPC: pseries: cpuidle: use time keeping flag

The current code computes the idle time but that can be handled
by the cpuidle framework if we enable the .en_core_tk_irqen flag.

Set the flag and remove the code related to the time computation.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Deepthi Dharwar <deepthi@linux.vnet.ibm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>