MAINTAINERS: Add a maintainer and collaborator for Silabs Platforms

Add Johan, Jérôme and Aksel to Silabs Platforms. Also update the subsystem
and GitHub label name from "SiLabs" to "Silabs", since this is the more
usual spelling.

Signed-off-by: Aksel Mellbye <aksel.mellbye@silabs.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Signed-off-by: Johan Hedberg <johan.hedberg@silabs.com>