There are devices in the wild with non-updatable firmware coming with
ACPI tables with rejected compatibles, e.g. "ltr,ltrf216a".  Linux
kernel still supports this device via ACPI PRP0001, however the
compatible was never accepted to bindings.

There are also several early PowerPC or SPARC platforms using
compatibles for their OpenFirmware, but without in-tree DTS.  Often the
legacy compatible is not correct in terms of current Devicetree
specification, e.g. missing vendor prefix.

Finally there are also Linux-specific tools and test code with
compatibles.

Add a schema covering above cases purely to satisfy the DT schema and
scripts/checkpatch.pl checks for undocumented compatibles.  For
ltr,ltrf216a this also documents the consensus: compatible is allowed
only via ACPI PRP0001, but not bindings.

Link: https://lore.kernel.org/all/20240705095047.90558-1-marex@denx.de/
Link: https://lore.kernel.org/lkml/20220731173446.7400bfa8@jic23-huawei/T/#me55be502302d70424a85368c2645c89f860b7b40
Cc: Marek Vasut <marex@denx.de>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240712121146.90942-1-krzysztof.kozlowski@linaro.orgSigned-off-by: Rob Herring (Arm) <robh@kernel.org>