ARM: dts: ixp4xx: Add PCI hosts

This adds a basic PCI host definition to the base device
tree for IXP4xx and then further details it in the 42x
and 43x device tree include, also the specific target
devices NSLU2 and GW2358 get proper PCI swizzling
defined.

Cc: Zoltan HERPAI <wigyori@uid0.hu>
Cc: Raylynn Knight <rayknight@me.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>