misc: Update MAINTAINERS with a system tag

misc: Update MAINTAINERS with a system tag

The code in system/ currently doesn't have a well-defined tag. Arm
components have traditionally used 'system-arm' and in some cases just
'arm'. This change introduces a separate tag for system software and
related files components.

Change-Id: I5c0ecd057bd65ab9d88c25f7ec7572d6941e41b5
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/19931
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>