[POWERPC] 85xx: Add 8568 PHY workarounds to board code

The 8568 MDS needs some configuration changes to the PHY in order to
work properly.  These are done in the firmware, normally, but Linux
shouldn't need to rely on the firmware running such things (someone
could disable the PHY support in the firmware to save space, for instance).

Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>