net: ftgmac100: Select ASPEED MDIO driver for the AST2600

Ensures we can talk to a PHY via MDIO on the AST2600, as the MDIO
controller is now separate from the MAC.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>