PNP devices may represent ACPI-enumerated hardware but do not have a
parent type supported by path_id. Consequently, importing path_id fails
even when the PNP device exposes a stable ACPI firmware_node. This also
prevents later assignments in rules such as the systemd-backlight
activation rule from taking effect.
Resolve the PNP device's firmware_node and use its ACPI sysname for the
path component. This gives PNP-backed devices the same stable identity
as their firmware representation.
Add a regression test using an RTC device below a PNP parent with an
ACPI firmware node.
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