ARM: ixp4xx: Add device tree boot support

This adds a minimal support for booting IXP4xx systems
from device tree.

We have to add hacks to the QMGR, NPE and notably also
ethernet and watchdog drivers so that they don't crash
the platform: these drivers are unconditionally starting
to grab regions of statically remapped IO space with no
concern of the device model or other platforms.

We will go in and properly fix these drivers as we go
along but for now this hack gets us to a place where we
can start working on proper device tree support for these
platforms.

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