ARM: kirkwood: Convert mv88f6281gtw_ge switch setup to DT

The mv88f6281gtw_ge has a ethernet switch connected to the ethernet
port of the SoC. Convert the platform device instantiation to a DT
instantiation.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Cc: florian@openwrt.org
Signed-off-by: Jason Cooper <jason@lakedaemon.net>