checkpatch: introduce proper bindings license check

All headers from 'include/dt-bindings/' must be verified by checkpatch
together with Documentation bindings, because all of them are part of the
whole DT bindings system.

The requirement is dual licensed and matching patterns:
* Schemas:
    /GPL-2\.0(?:-only)? OR BSD-2-Clause/
* Headers:
    /GPL-2\.0(?:-only)? OR \S+/

Above patterns suggested by Rob at:
https://lore.kernel.org/all/CAL_Jsq+-YJsBO+LuPJ=ZQ=eb-monrwzuCppvReH+af7hYZzNaQ@mail.gmail.com

The issue was found during patch review:
https://lore.kernel.org/all/20230313201259.19998-4-ddrokosov@sberdevices.ru/

Link: https://lkml.kernel.org/r/20230404191715.7319-1-ddrokosov@sberdevices.ru


Signed-off-by: Dmitry Rokosov <ddrokosov@sberdevices.ru>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Dwaipayan Ray <dwaipayanray1@gmail.com>
Cc: Joe Perches <joe@perches.com>
Cc: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>