cpupower: Initial AMD P-State capability

If kernel starts the AMD P-State module, the cpupower will initial the
capability flag as CPUPOWER_CAP_AMD_PSTATE. And once AMD P-State
capability is set, it won't need to set legacy ACPI relative
capabilities anymore.

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