cpufreq: amd-pstate: Add test module for amd-pstate driver

Add amd-pstate-ut test module, this module is used by kselftest
to unit test amd-pstate functionality. This module will be
expected by some of selftests to be present and loaded.

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