ACPI / enumeration: Document the rules regarding the PRP0001 device ID

Document how the ACPI device enumeration code uses the special
PRP0001 device ID.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Hanjun Guo <hanjun.guo@linaro.org>
Reviewed-by: Darren Hart <dvhart@linux.intel.com>