MAINTAINERS: Add entry to MAINTAINERS for Milbeaut

Add entry to MAINTAINERS for Milbeaut that supported minimal drivers.

Signed-off-by: Sugaya Taichi <sugaya.taichi@socionext.com>
Link: https://lore.kernel.org/r/1636968656-14033-5-git-send-email-sugaya.taichi@socionext.com

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