drm/amdgpu/pm: report gpu voltages via hwmon API

Expose vddgfx and vddnb via hwmon.

Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>