TSEC driver: Change MDIO support to allow access to any PHYs on the MDIO bus

The current TSEC driver limits MDIO access to the devices that have been configured as attached
to a TSEC MAC. This patch allows access to any PHY device on the MDIO bus through the 'mii' commands.

Signed-off-by: Michael Firth <michael.firth@bt.com>
Acked-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>