drm/amdgpu/swsmu: implement voltage metrics for RENOIR

Grab the data from the SMU metrics table.

Reviewed-by: Evan Quan <evan.quan@amd.com>
Acked-by: Nirmoy Das <nirmoy.das@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>