Added a stress test for cpufreq based on libjpeg-turbo jpeg decoding

We are testing all the available cpufreq operating points and checking
whether jpeg decoding produces exactly the same result across different
runs. It just happens that libjpeg-turbo is a rather tough workload,
which tends to expose some overclocking/stability issues:
    https://www.mail-archive.com/linux-sunxi@googlegroups.com/msg04343.html

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>