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>
Merge with driver
Merge with dtsi
Merge with hackberry
Merge with cubie
Merge with driver
Merge with mdio
Merge with driver
Work on NAPI