selftests: amd-pstate: Add test trigger for amd-pstate driver

Add amd-pstate test trigger in kselftest, it will load/unload
amd-pstate-ut module to test some cases etc.

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>