Add rc6_residency_counter subtest

Add rc6_residency_counter subtest

Move rc6_residency_check to subtest, add new rc6_residency_counter subtest
for pm_rc6_residency IGT case.

Test results run on platforms show as below:
On HSW
---------------------------------------
[root@x-hswu opt]# ./pm_rc6_residency
IGT-Version: 1.6-g35b31df (x86_64) (Linux: 3.15.0-rc3_drm-intel-nightly_0791a3_20140520+ x86_64)
Subtest rc6-residency-check: SUCCESS
This machine doesn't support rc6pp
This machine doesn't support rc6p
This machine entry  rc6 status.
The residency counter : 0.999667
Subtest rc6-residency-counter: SUCCESS

On IVB
----------------------------------------
[root@IVB tests]# ./pm_rc6_residency
IGT-Version: 1.6-g35b31df (x86_64) (Linux: 3.13.6_20140318+ x86_64)
Subtest rc6-residency-check: SUCCESS
This machine entry  rc6p status.
The residency counter : 0.997000
Subtest rc6-residency-counter: SUCCESS

On BYT
----------------------------------------
root@x-byt:/opt# ./pm_rc6_residency
IGT-Version: 1.6-g0d39021 (x86_64) (Linux: 3.14.0_kcloud_ceabbb_20140521+ x86_64)
Subtest rc6-residency-check: SUCCESS
This machine doesn't support rc6pp
This machine doesn't support rc6p
The residency counter : 1.144333
Test assertion failure function rc6_residency_counter, file pm_rc6_residency.c:131:
Last errno: 0, Success
Failed assertion: counter_result <=1
Debug files must be wrong,
Subtest rc6-residency-counter: FAIL

On BDW
---------------------------------------
[root@x-bdw01 opt]# ./pm_rc6_residency
IGT-Version: 1.6-g0d39021 (x86_64) (Linux: 3.15.0-rc5_drm-intel-nightly_367653_20140521+ x86_64)
Subtest rc6-residency-check: SUCCESS
This machine doesn't support rc6pp
This machine doesn't support rc6p
The residency counter : 0.994333
This machine entry rc6 state.
Subtest rc6-residency-counter: SUCCESS

Signed-off-by: Liu, Lei A <lei.a.liu@intel.com>
Signed-off-by: Wendy Wang <wendy.wang@intel.com>
Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com>