These Gemini boards have Ethernet PHY on GPIO bit-banged
MDIO, clearly defined in the corresponding OpenWRT
ethernet patches since ages. Add them in accordance with
the OpenWRT patch so we can use them when we add ethernet
support.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>