ARM: kirkwood: Add support for RaidSonic IB-NAS6210/6220 using devicetree

Add support for the IB-NAS6210 and IB-NAS 6220. Describe as much as
currently possible in the devicetree files, including the NAND partitions.
Use the partition scheme of the original firmware by default.

Create a board-ib62x0.c for everything else.

Signed-off-by: Simon Baatz <gmbnomis@gmail.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>