ACPI / platform: fix comment about the platform device name

We don't use _UID anymore, instead the name will be taken from the
corresponding ACPI device (adev). Fix the obsolete comment.

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