net: Add MDIO bus driver for the Allwinner EMAC

This patch adds a separate driver for the MDIO interface of the
Allwinner ethernet controllers.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Tested-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>