MAINTAINERS: Add Gardena maintainer and collaborators

Adding myself as maintainer because my employer Husqvarna wants to
upstream the Gardena smart gateway radio module SoM to Zephyr, and keep
it well-supported.

Adding as collaborator M1cha, because he did the initial port.

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