km/spi: activate driver model for SPI flash on KM Kirkwood boards

The corresponding configurations are selected in the common Kconfig file.
This is easier than changing every affected board default configuration
file. The default configuration for the PORTL2 board, however, still needs
some modifications to correctly use the driver model.

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>