ACPI: Support Generic Initiator only domains

Generic Initiators are a new ACPI concept that allows for the
description of proximity domains that contain a device which
performs memory access (such as a network card) but neither
host CPU nor Memory.

This patch has the parsing code and provides the infrastructure
for an architecture to associate these new domains with their
nearest memory processing node.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>