cpupower: Add EPP value change support

amd_pstate and intel_pstate active mode drivers support energy
performance preference feature. Through this user can convey it's
energy/performance preference to platform. Add this value change
capability to cpupower.

To change the EPP value use below command:
cpupower set --epp performance

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