ARM: kirkwood: add db-88f6281-bp board

This is Marvell's Kirkwood development board. It has the following
features

 - 512M DDR2
 - 2 PCI connectors
 - 1 x1 PCI-e interface
 - 1 Gigabit Ethernet Port
 - 2 SATA Ports
 - USB 2.0 Interface
 - SDIO
 - 128M NAND Flash
 - 16M SPI Flash

It can be strapped to boot from SPI or NAND so there are two defconfigs
(one per boot media).

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Stefan Roese <sr@denx.de>