MAINTAINERS: Add SiM3U maintainer and collaborators

Adding myself as maintainer because my employer Husqvarna wants to
upstream SiM3U support to Zephyr, and keep it well-supported.

Adding as collaborator M1cha, because he did the initial port and
Asmellby, jerome-pouiller, and jhedberg because they take care of
all Silabs products.

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>