net: macb: Use mdio child node for MDIO bus if it exists

This allows explicitly specifying which children are present on the mdio
bus. Additionally, it allows for non-phy MDIO devices on the bus.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>