net: dsa: mv88e6xxx: scratch registers and external MDIO pins

MV88E6352 and later switches support GPIO control through the "Scratch
& Misc" global2 register. Two of the pins controlled this way on the
mv88e6390 family are the external MDIO pins. They can either by used
as part of the MII interface for port 0, GPIOs, or MDIO. Add a
function to configure them for MDIO, if possible, and call it when
registering the external MDIO bus.

Suggested-by: Russell King <rmk@armlinux.org.uk>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>