ARM: kirkwood: Basic support for DNS-320 and DNS-325

Add support for the DNS-320 and DNS-325. Describe as much as currently possible
in the devicetree files, create a board-dnskw.c for everything else.

Changes since last submission (V3) [Addressing comments by]:-

* One MACH_DLINK_KIRKWOOD_DT for all dtb files [Grant Likely, Jason Cooper]
* Drop brain-dead select "select CONFIG_MTD_OF_PARTS" [Grant Likely]
* Don't add NAND support then throw it away immediately after [Grant Likely]
* Describe purpose of MPP 41, 42 & 49

Changes since last submission (V2):-

* Use IEEE-compliant "okay", rather than "ok" [Scott Wood]

Signed-off-by: Jamie Lentin <jm@lentin.co.uk>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>