scripts/checkpatch.pl: Resync with v5.16

This resyncs us with the version found in v5.16 of the Linux kernel with
the following exceptions:
- Keep our u-boot specific tests / code area.
- Change the location of checkpatch.rst
- Drop the "use strscpy" test as we don't have that, but do have strlcpy
  and want that used now.
- Keep debug/printf in the list for $logFunctions

This also syncs the spdxcheck.py tool and all the associated
documentation.
S
Signed-off-by: Simon Glass <sjg@chromium.org>