ixp4xx: support omicron ixp425 based boards

This patch adds board support for the DEVIXP, the MICCPT, and the
MIC256, which are three IXP425 based boards produced by OMICRON
electronics, GmbH.

Signed-off-by: Richard Cochran <richard.cochran@omicron.at>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>