net: mvmdio: new Marvell MDIO driver

This patch adds a separate driver for the MDIO interface of the
Marvell Ethernet controllers. There are two reasons to have a separate
driver rather than including it inside the MAC driver itself:

 *) The MDIO interface is shared by all Ethernet ports, so a driver
    must guarantee non-concurrent accesses to this MDIO interface. The
    most logical way is to have a separate driver that handles this
    single MDIO interface, used by all Ethernet ports.

 *) The MDIO interface is the same between the existing mv643xx_eth
    driver and the new mvneta driver. Even though it is for now only
    used by the mvneta driver, it will in the future be used by the
    mv643xx_eth driver as well.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: David S. Miller <davem@davemloft.net>