arm/km: use spi claim bus to switch between SPI and NAND

We overwrite these weak functions from the kirkwood spi code to
use our own method to be able to switch between the SPI NOR and
the NAND flash. This is needed e.g. to update the u-boot. The former
command do_spi_toggle can therefore be removed. And the usage of
this command is removed from the u-boot update command in the
u-boot environment.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Signed-off-by: Prafulla Wadaskar <prafulla@marvell.com>
cc: Holger Brunck <holger.brunck@keymile.com>
cc: Prafulla Wadaskar <prafulla@marvell.com>