hostnamed: use /proc/device-tree to get chassis type

hostnamed: use /proc/device-tree to get chassis type

From https://www.kernel.org/doc/Documentation/ABI/testing/sysfs-firmware-ofw
---
Userspace must not use the /sys/firmware/devicetree/base
path directly, but instead should follow /proc/device-tree
symlink. It is possible that the absolute path will change
in the future, but the symlink is the stable ABI.
---

Addresses the comment https://github.com/systemd/systemd/pull/20731#discussion_r744095262.