Support 1000base-x mode on Topaz on 4.14 kernel
This adds support for 1000base-x mode on Topaz' SerDes port (when defined in DTS).
1000base-x is currently needed for some presentations and also to fix JJJJ issue for users wanting to fix this by lowering SerDes speed.
The mode has to be defined via device tree.
Code changes are from upstream kernel.