drm/amdgpu: add support on mmhub for navy_flounder

navy_flounder has the same mmhub IP version with sienna_cichlid,
follow its setting.

Signed-off-by: Jiansong Chen <Jiansong.Chen@amd.com>
Reviewed-by: Tao Zhou <Tao.Zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>