LICENSES: add dual/CC-BY-4.0

The license CC-BY-4.0 is not compatible with GPLv2, but files can
be dual licensed with a GPLv2 compatible license 'OR' CC-BY-4.0.

This is the case for some file auto-generated by riscv project.

Change-Id: I4313d85a569a5e6423392129a730d1e22ef17c51
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8894
Reviewed-by: Bernhard Rosenkraenzer <bero@baylibre.com>
Tested-by: jenkins

tools/scripts/spdxcheck.py

@@ -50,7 +50,7 @@ def read_spdxdata(repo):
     # The subdirectories of LICENSES in the kernel source
     # Note: exceptions needs to be parsed as last directory.
     # OpenOCD specific: Begin
-    license_dirs = [ "preferred", "stand-alone", "exceptions" ]
+    license_dirs = [ "preferred", "dual", "exceptions", "stand-alone" ]
     # OpenOCD specific: End
     lictree = repo.head.commit.tree['LICENSES']