[ARM] Kirkwood: properly handle the WAN port on newer RD88F6281 boards

On newer versions of the RD88F6281 board, the WAN port is connected to
its own ethernet port on the CPU, via a separate PHY, whereas on older
versions of the board, it is connected to one of the PHYs in the
ethernet switch.  In the RD8F6281 setup code, detect which version of
the board we are running on, and instantiate the ethernet ports and
switch driver accordingly.

Signed-off-by: Ronen Shitrit <rshitrit@marvell.com>
Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: Nicolas Pitre <nico@marvell.com>