Add new features - an MDIO clock, WMAC reset, GMAC reset and ethernet

Add new features - an MDIO clock, WMAC reset, GMAC reset and ethernet
switch reset/initialise functions.

The AR934x and QC955x SoCs both have a configurable MDIO base clock.
The others have the MDIO clock use the same clock as the system
reference clock, whatever that may be.

Tested:

* AR9344 SoC

TODO:

* mips24k - AR933x would be fine for now, just to ensure that things
  are sane.