Only certain types of ACPI device objects can be enumerated as
platform devices, so in order to distinguish them from the others
introduce a new ACPI device PNP type flag, platform_id, and set it
for devices with a valid _HID to start with.

This change is based on a Zhang Rui's prototype.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>