ACPI / Documentation: refer to correct file for acpi_platform_device_ids[] table

When the ACPI platform device code was converted to the new ACPI scan
handler facility, the the acpi_platform_device_ids[] was moved to
drivers/acpi/acpi_platform.c. Update the documentation accordingly.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>