ARM: dts: sunxi: h3/h5: represent the mdio switch used by sun8i-h3-emac

Since dwmac-sun8i could use either an integrated PHY or an external PHY
(which could be at same MDIO address), we need to represent this selection
by a MDIO switch.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>