Pull in SPDX tag conversion from upstream dtc. This will replace the
conversion done in the kernel tree copy in v5.2-rc2.

This adds the following commits from upstream:

702c1b6c0e73 README.license: Update to reflect SPDX tag usage
4097bbffcf1d dtc: Add GPLv2 SPDX tags to files missing license text
94f87cd5b7c5 libfdt: Add dual GPL/BSD SPDX tags to files missing license text
c4ffc05574b1 tests: Replace license boilerplate with SPDX tags
a5ac29baacd2 pylibfdt: Replace dual GPLv2/BSD license boilerplate with SPDX tags
7fb0f4db2eb7 libfdt: Replace GPL/BSD boilerplate/reference with SPDX tags
acfe84f2c47e dtc: Replace GPLv2 boilerplate/reference with SPDX tags
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Rob Herring <robh@kernel.org>