dt-bindings: vendor-prefixes: Add "test" vendor for KUnit and friends

Add the vendor prefix "test" to reserve a vendor prefix for bindings
that are purely for testing device tree code. This allows test code to
write bindings that can be checked by the schema validator.

Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: David Gow <davidgow@google.com>
Cc: Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>
Cc: Conor Dooley <conor+dt@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20240718210513.3801024-4-sboyd@kernel.org