With the new `bootc install to-filesystem` support many stages
will need a devices/mount setup to bind mount the deployment root
from the bootc deployment root of the generated image. To make
this globally available just allow "devices/mounts" for all stages
in the schema validation.
Note that `mounts` is already globally allowed so this just adds
devices (this was added in `7e776a076` with ostree as the use-case).
Nothing will change for the filesystem stages that already define
"devices" in a more specialized way.
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