fs_enet: Add MPC5125 FEC support and PHY interface selection

Add compatible string for MPC5125 FEC. The FEC on MPC5125 additionally
supports RMII PHY interface. Configure controller/PHY interface type
according to the optional phy-connection-type property in the ethernet
node. This property should be either "rmii" or "mii".

Signed-off-by: Vladimir Ermakov <vooon341@gmail.com>
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>