selftests/cpufreq: Rename DEBUG_PI_LIST to DEBUG_PLIST

DEBUG_PI_LIST was renamed to DEBUG_PLIST since
8e18faea ("lib/plist: rename DEBUG_PI_LIST to DEBUG_PLIST")

- It's not reasonable to keep the deprecated configs.
- configs under kselftests are recommended by corresponding tests.
So if some configs are missing, it will impact the testing results

CC: "Rafael J. Wysocki" <rjw@rjwysocki.net>
CC: Viresh Kumar <viresh.kumar@linaro.org>
CC: linux-pm@vger.kernel.org
Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>