km/rgmii: port Ethernet interface of KM Kirkwood boards to driver model

Activate the driver model for the Ethernet interface (RGMII) in the KM
Kirkwood Kconfig file. Additionally, raise the auto negotiation timeout
to eight seconds as more time is required for those boards.

Signed-off-by: Pascal Linder <pascal.linder@edu.hefr.ch>
Signed-off-by: Holger Brunck <holger.brunck@ch.abb.com>
Signed-off-by: Stefan Roese <sr@denx.de>