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>

Author: rockosov

scripts/checkpatch.pl

@@ -3763,7 +3763,7 @@ sub process {
 						     "'$spdx_license' is not supported in LICENSES/...\n" . $herecurr);
 					}
 					if ($realfile =~ m@^Documentation/devicetree/bindings/@ &&
-					    not $spdx_license =~ /GPL-2\.0.*BSD-2-Clause/) {
+					    $spdx_license !~ /GPL-2\.0(?:-only)? OR BSD-2-Clause/) {
 						my $msg_level = \&WARN;
 						$msg_level = \&CHK if ($file);
 						if (&{$msg_level}("SPDX_LICENSE_TAG",
@@ -3773,6 +3773,11 @@ sub process {
 							$fixed[$fixlinenr] =~ s/SPDX-License-Identifier: .*/SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)/;
 						}
 					}
+					if ($realfile =~ m@^include/dt-bindings/@ &&
+					    $spdx_license !~ /GPL-2\.0(?:-only)? OR \S+/) {
+						WARN("SPDX_LICENSE_TAG",
+						     "DT binding headers should be licensed (GPL-2.0-only OR .*)\n" . $herecurr);
+					}
 				}
 			}
 		}