km/spi: overwrite kirkwood_spi weak functions for KM Kirkwood boards

As the SPI NOR and NAND devices share the same hardware pins, the MPP
configuration has to be changed when claiming/releasing the bus. The
current configuration is saved when claiming and restored when releasing.
Furthermore, a general-purpose output is used to switch the chip-select
signal. This is now also implemented for the DM part of the kirkwood_spi
driver.

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>