IXP4xx MAINTAINERS entries

Update MAINTAINERS entries for Intel IXP4xx SoCs.

Linus has been handling all IXP4xx stuff since 2019 or so.

Signed-off-by: Krzysztof Hałasa <khalasa@piap.pl>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Deepak Saxena <dsaxena@plexity.net>
Link: https://lore.kernel.org/r/m3ttqxu4ru.fsf@t19.piap.pl


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