[POWERPC] FSL: enet device tree cleanups

* Removed address fields in ethernet nodes
* Removed #address-cells, #size-cells from gianfar nodes
* Added cell-index to gianfar and ucc ethernet nodes
* Added enet[0..3] labels
* Renamed compatible node for gianfar mdio to "fsl,gianfar-mdio"
* Removed device_type = "mdio"

The matching for gianfar mdio still supports the old "mdio"/"gianfar" combo
but it is now considered deprecated.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>