km/spi: add weak functions to kirkwood_spi driver (DM part)

The weak functions, known from the legacy code, are added to the DM part
as well. For this purpose, the release operation first needs to be
implemented. KM Kirkwood boards will overwrite those weak functions to
change the MPP configuration when claiming/releasing the bus, because the
hardware pins are shared between the SPI NOR and NAND devices.

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>