selftests: amd-pstate: Trigger tbench benchmark and test cpus

Add tbench.sh trigger the tbench testing and monitor the cpu desire
performance, frequency, load, power consumption and throughput etc.

Signed-off-by: Meng Li <li.meng@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>