drm/amd/powerplay: add atomctrl_get_voltage_evv function in ppatomctrl

The atomctrl_get_voltage_evv function will be used on iceland HW
manager.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>