Do not attach com at acpi when there's no address or irq present. Fixes

Do not attach com at acpi when there's no address or irq present. Fixes
a regression caused by the recent change to start attaching com at acpi
as it turns out that Libreboot exposes console devices lacking crucial
data in their acpi tables. The same console attaches fine over isa,
therefore restore this behavior.

Problem reported by <cipher-hearts at riseup dot net> on bugs@

ok deraadt@ kettenis@