km/spi: remove deprecated SPI flash driver code for KM Kirkwood boards

KM Kirkwood boards now implement the driver model for its SPI flash
interface. Therefore, the old board specific claim and release functions
can be deleted. The preprocessor definition CONFIG_SYS_KW_SPI_MPP is yet
unused as well. All its appearances and dependencies are removed in the
kirkwood_spi driver, header files and finally the configuration whitelist.

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>