kernel/arm: fix system_time for generic timer

Change-Id: Ief7b0638f40ba5deee27f3551e8870bfb6216a4d
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5569
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Fredrik Holmqvist <fredrik.holmqvist@gmail.com>

kernel/arm: implement generic timer

Change-Id: I5edb7e36013d5e5d9d45ec3cf31014f99601a4c6
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5519
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Fredrik Holmqvist <fredrik.holmqvist@gmail.com>