net: dsa: mv88e6xxx: Add minimal platform_data support

Not all the world uses device tree. Some parts of the world still use
platform devices and platform data. Add basic support for probing a
Marvell switch via platform data.

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>