acpi(4): Add _CID to PNP info string

While a given ACPI device may have 0-N compatibility IDs, in practice most
seem to have 0 or 1. If one is present, emit it as part of the PNP info
string associated with a device. This could enable MODULE_PNP_INFO-based
automatic kldload for ACPI drivers associated with a given _CID (but without
a good _HID or _UID identifier).

Reviewed by: imp, jhb
Differential Revision: https://reviews.freebsd.org/D22846