[ARM] Kirkwood: allow instantiating the second ethernet port

The 88f6192 and 88f6281 Kirkwood SoCs support two ethernet ports.
Add the platform glue that will allow board support files to
instantiate the second ethernet port.

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