ACPI: scan: Obtain device's desired enumeration power state

Store a device's desired enumeration power state in struct
acpi_device_power during acpi_device object's initialisation.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>