selftests: cpufreq: conform test to TAP

This test outputs lots of information. Let's conform the core part of
the test to TAP and leave the information printing messages for now.
Include ktap_helpers.sh to print conformed logs. Use KSFT_* macros to
return the correct exit code for the kselftest framework and CIs to
understand the exit status.

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>