dt-bindings: net: add marvell usb to mdio bindings

Describe how the USB to MDIO controller can optionally use device tree
bindings to reference attached devices such as switches.

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