Allow the sio acpi attachment to be disabled (ie: use hints only). This

Allow the sio acpi attachment to be disabled (ie: use hints only).  This
hack means you can get the units and flags to match up more easily with
serial consoles on machines with acpi tables that cause the com ports
to be probed in the wrong order (and hence get the wrong sio unit number).

This replaces the common alternative hack of editing the code to comment
out the acpi attachment.  This could go away entirely when device wiring
patches are committed.