stmmac: create of compatible mdio bus for stmmac driver

The DSA driver needs to be passed a reference to an mdio bus. Typically
the mac is configured to use a fixed link but the mdio bus still needs
to be registered so that it con configure the switch.
This patch follows the same process as the altera tse ethernet driver for
creation of the mdio bus.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>