cpupower: Add support for amd_pstate mode change

amd_pstate supports changing of its mode dynamically via `status` sysfs
file. Add the same capability in cpupower. To change the mode to active
mode use below command:

cpupower set --amd-pstate-mode active

Acked-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Gautham R. Shenoy <gautham.shenoy@amd.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Wyes Karny <wyes.karny@amd.com>
Tested-by: Perry Yuan <Perry.Yuan@amd.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>