ACPI / scan: Add labels for PNP button devices

Subsequent code treats button_device_ids[] entries differently, and
it's hard to follow without a hint as to which is which.

Add comments to identify the power button, lid, and sleep button
devices.

The "PNP" prefix is owned by Microsoft, so they distribute the
canonical list of "PNP" IDs.

Link: https://uefi.org/PNP_ACPI_Registry
Link: https://download.microsoft.com/download/1/6/1/161ba512-40e2-4cc9-843a-923143f3456c/devids.txt


Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>