net: phy: Marvell: Add mv88e6390 internal PHY

The mv88e6390 Ethernet switch has internal PHYs. These PHYs don't have
an model ID in the ID2 register. So the MDIO driver in the switch
intercepts reads to this register, and returns the switch family ID.
Extend the Marvell PHY driver by including this ID, and treat the PHY
as a 88E1540.

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