selftest: cpufreq: Add support for cpufreq tests

This patch adds supports for basic cpufreq tests, which can be performed
independent of any platform.

It does basic tests for now, like
- reading all cpufreq files
- trying to update them
- switching frequencies
- switching governors

This can be extended to have more specific tests later on.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>