drm/amd/powerplay: add new clock type in struct gfx_arbit.

Add the new clock type to the gfx arbitor so we can determine
the proper clock floors for it.

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>