powerpc/selftest: Add support for cpuidle latency measurement

The cpuidle latency selftest provides support to systematically extract,
analyse and present IPI and timer based wakeup latencies for each CPU
and each idle state available on the system.

The selftest leverages test_cpuidle_latency module's debugfs interface
to interact and extract latency information from the kernel.

The selftest inserts the module if already not inserted, disables all
the idle states and enables them one by one testing the following:

1. Keeping source CPU constant, iterate through all the cores and pick
   a single CPU for each core measuring IPI latency for baseline
   (CPU is busy with cat /dev/random > /dev/null workload) and then
   when the CPU is idle.
2. Iterating through all the CPU cores and selecting one CPU for each
   core, then, the expected timer durations to be equivalent to the
   residency of the deepest idle state enabled is sent to the selected
   target CPU, then the difference between the expected timer duration
   and the time of wakeup is determined.

To run this test specifically:
$ sudo make -C tools/testing/selftests \
  TARGETS="powerpc/cpuidle_latency" run_tests

There are a few optional arguments too that the script can take
        [-h <help>]
        [-i <run timer tests>]
        [-m <location of the module>]
        [-s <source cpu for ipi test>]
        [-o <location of the output>]
        [-v <verbose> (run on all cpus)]

Default Output location in:
tools/testing/selftests/powerpc/cpuidle_latency/cpuidle_latency.log

To run the test without re-compiling:
$ cd tools/testing/selftest/powerpc/cpuidle_latency/
$ sudo ./cpuidle_latency.sh

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>