net: phy: add marvell usb to mdio controller

An MDIO controller present on development boards for Marvell switches
from the Link Street (88E6xxx) family.

Using this module, you can use the following setup as a development
platform for switchdev and DSA related work.

   .-------.      .-----------------.
   |      USB----USB                |
   |  SoC  |      |  88E6390X-DB  ETH1-10
   |      ETH----ETH0               |
   '-------'      '-----------------'

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>