MAINTAINERS: Add myself as MStar/Sigmastar Armv7 SoC maintainers

As proposed by Daniel, I am going to help him to maintain the platform,
so add myself as co-maintainer.

Signed-off-by: Romain Perier <romain.perier@gmail.com>
Acked-by: Daniel Palmer <daniel@thingy.jp>
Link: https://lore.kernel.org/r/20210910190322.27058-5-romain.perier@gmail.com

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