cpupower: Add the function to check AMD P-State enabled

The processor with AMD P-State function also supports legacy ACPI
hardware P-States feature as well. Once driver sets AMD P-State eanbled,
the processor will respond the finer grain AMD P-State feature instead of
legacy ACPI P-States. So it introduces the cpupower_amd_pstate_enabled()
to check whether the current kernel enables AMD P-State or AMD CPUFreq
module.

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