drm/amd/powerplay: define an universal data structure for gpu metrics (V4)

Thus we can provide an interface for UMD to retrieve gpu metrics data.

V2: better naming and comments
V3: two structures created for dGPU and APU separately
V4: add driver attached timestamp

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