MFC r326956 (by cem):
Implement ACPI CPU support when Processor object is not present

By the ACPI standard (ACPI 5 chapter 8.4 Declaring Processors) Processors
can be implemented in 2 distinct ways:

• Through a Processor object type (which provides P_BLK)
• Through a Device object type

Prior to this change, the FreeBSD driver only supported the former. AMD
Epyc / Poweredge systems we are testing both implement the latter only. Add
the missing support.

Because P_BLK is not defined in the device object case, C-states entering
must be completely controlled via _CST methods rather than P_LVL2/3.

John Baldwin points out that ACPI 6.0 formally deprecates the Processor
keyword, so eventually processors will only be enumerated as Device objects.