Documentation: amd-pstate: Add unit test introduction

Introduce the AMD P-State unit test module design and implementation.
It also talks about kselftest and how to use.

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>