Add AP121 ethernet / switch PHY support.

* arge0 is MII
* arge1 is GMII
* the MDIO bus is on arge1, not arge0
* the default switch config is to have ports 0-3 as the switch group,
  with port 4 being an external PHY dedicated to arge0 (ie, 'cpu' port.)

Whilst I'm here, remove unused bits and pieces from the config.

Tested:

* AP121, ping on both arge0 and arge1
* Tested switch port detection using etherswitchcfg

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