devicetree: edtlib: prefixes which are not vendors are NOT vendors

In Linux, checkpatch.pl relies on the vendor-prefixes.yaml file
to validate manufacturers in compatible strings.
In addition to the vendors defined in vendor-prefixes.txt,
the YAML file includes expressions for "prefixes which are not vendors":
these expressions do NOT define special manufacturers that may appear
in compatible strings, and are never involved as such in DTS files.
We can rather see them as bulk-definitions of JSON/YAML properties
suitable for the dt-schema tools.

OTHO, in Zephyr, checkpatch.pl relies on the vendor-prefixes.txt file,
which does not include these additional prefixes, but edtlib.EDT adds
them as hard-coded special values.

This is confusing, if not incorrect:

- the fact that edtlib.EDT (and thus its client code in the
zephyr/scripts directory) actually allows these vendors
in compatible strings is buried in the source code
- checkpatch.pl (with vendor-prefixes.txt) in Zephyr behaves neither like
checkpatch.pl (with vendor-prefixes.yaml) in Linux, nor like edtlib.EDT
(with _VENDOR_PREFIX_ALLOWED)
- Zephyr should not treat these "prefixes which are not vendors" as
valid manufacturers in compatible strings to begin with

Signed-off-by: Christophe Dufaza <chris@openmarl.org>