Update the AR934x SoC support.

* Add the MDIO clock probe during clock initialisation;
* Update the ethernet PLL configuration function to use the correct
  values;
* Add a GMAC block configuration to pull the configuration out of hints;
* Add an ethernet switch reconfiguration method.

Tested:

* AR9344 SoC (DB120)

.. however, this has been tested with extra patches in my tree (to fix
the ethernet/MDIO support, SPI support, ethernet switch support)
and thus it isn't enough to bring the full board support up.
Add the long-missing spibus_if.m to the MFILES list.