powerpc/cpuidle: cpuidle wakeup latency based on IPI and timer events

Introduce a mechanism to fire directed IPIs from a source CPU to a
specified target CPU and measure the time incurred on waking up the
target CPU in response.

Also, introduce a mechanism to queue a hrtimer on a specified CPU and
subsequently measure the time taken to wakeup the CPU.

Define a simple debugfs interface that allows for adjusting the
settings to trigger IPI and timer events on a designated CPU, and to
observe the resulting cpuidle wakeup latencies.

Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Pratik R. Sampat <psampat@linux.ibm.com>
Signed-off-by: Aboorva Devarajan <aboorvad@linux.vnet.ibm.com>