ARM: dts: Add some initial IXP4xx device trees

This adds a device tree for the IXP4xx-based Linksys
NSLU2 and Gateworks GW2358 which encompass the Gateworks
Cambria family.

These will be the first IXP4xx device tree platforms.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>