drivers: net: add marvell MDIO driver

This patch adds a separate driver for the MDIO interface of the
Marvell Ethernet controllers based on driver model. 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/mvpp2 driver. Even though it is for now
     only used by the mvneta/mvpp2 driver, it will in the future be
     used by the mv643xx_eth driver as well.

This driver supports SMI IEEE for 802.3 Clause 22 and XSMI for IEEE
802.3 Clause 45.

This patch also adds device tree binding for marvell MDIO driver.

Signed-off-by: Ken Ma <make@marvell.com>
Signed-off-by: Alex Marginean <alexm.osslist@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>