system/kernel: Rework uart management

Change-Id: I6cb31760519c8ba4542d217d6e68439602eda558
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4356
Reviewed-by: Jessica Hamilton <jessica.l.hamilton@gmail.com>
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>