ARM: kirkwood: disable dcache for Kirkwood boards

Prior to commit 93b283d4 ("ARM: CPU: arm926ejs: Consolidate cache
routines to common file") the kirkwood boards didn't have and dcache
support. The network and usb drivers rely on this. Set
CONFIG_SYS_DCACHE_OFF in the Kirkwood specific config.h.

Reported-by: Leigh Brown <leigh@solinno.co.uk>
Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>