cpufreq: amd-pstate: Add explanation for X86_AMD_PSTATE_UT

This kernel module is used for testing. It's safe to say M here.
It can also be built-in without X86_AMD_PSTATE enabled.
Currently, only tests for amd-pstate are supported. If X86_AMD_PSTATE
is set disabled, it can tell the users test can only run on amd-pstate
driver, please set X86_AMD_PSTATE enabled.
In the future, comparison tests will be added. It can set amd-pstate
disabled and set acpi-cpufreq enabled to run test cases, then compare
the test results.

Suggested-by: Shuah Khan <skhan@linuxfoundation.org>
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>