MAINTAINERS: add related dts to IXP4xx

get_maintainer.pl currently give no maintainer on all IXP4xx DTS files.
Add them to the set of files handled by IXP4xx maintainers.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20221005105734.3513581-1-clabbe@baylibre.com
Link: https://lore.kernel.org/r/20221214204223.177807-1-linus.walleij@linaro.org


Signed-off-by: Arnd Bergmann <arnd@arndb.de>