ARM: kirkwood: add dreamplug (fdt) support.

Initially, copied guruplug-setup.c and did s/guruplug/dreamplug/g.
Then, switched to SPI based NOR flash.

After talking to Arnd Bergman, chose an incremental approach to adding
devicetree support.  First, we use the dtb to tell us we are on the
dreamplug, then we gradually port over drivers.

Driver porting will start with the uart (see next patch), and progress
from there.  Possibly, spi/flash/partitions will be next.

When done, board-dt.c will no longer be dreamplug specific, and dt's can
be made for the other kirkwood boards.

Signed-off-by: Jason Cooper <jason@lakedaemon.net>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>