Add support to read the _CLS entry if it's present. It is used by
memory-mapped devices that are normally PCIe drives. Devices can then use
the existing pci_get_class, etc. accessors to query this data.

The ivar values are different enough from the existing ACPI and ISA values
to not conflict.

Reviewed by: jhb
Obtained from: ABT Systems Ltd
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D8721