spi: kirkwood: add orion-spi compatible string

This matches the compatible string used by the Linux kernel. This will
allow u-boot to use the same device tree files.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>
[jagan: use armada instead of orion on .data]
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>