gianfar: Convert gianfar to an of_platform_driver

Does the same for the accompanying MDIO driver, and then modifies the TBI
configuration method.  The old way used fields in einfo, which no longer
exists.  The new way is to create an MDIO device-tree node for each instance
of gianfar, and create a tbi-handle property to associate ethernet controllers
with the TBI PHYs they are connected to.

Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>