net: stmmac: dwmac-sun8i: Handle integrated/external MDIOs

The Allwinner H3 SoC have two distinct MDIO bus, only one could be
active at the same time.
The selection of the active MDIO bus are done via some bits in the EMAC
register of the system controller.

This patch implement this MDIO switch via a custom MDIO-mux.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>