ibm_newemac: Cleanup/Fix RGMII MDIO support detection

More than just "AXON" version of EMAC RGMII supports MDIO, so replace
the current test with a generic property in the device-tree that
indicates such support.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Stefan Roese <sr@denx.de>
Signed-off-by: Jeff Garzik <jeff@garzik.org>