ftgmac100: Make the MDIO bus a child of the ethernet device

Populate mii_bus->parent with our own platform device before
registering, which makes it easier to locate the MDIO bus
in sysfs when trying to diagnose problems.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: David S. Miller <davem@davemloft.net>