scripts/checkpatch.pl: Resync with v5.13

scripts/checkpatch.pl: Resync with v5.13

This resyncs us with the version found in v5.13 of the Linux kernel with
the following exceptions:
- Keep our u-boot specific tests / code area.
- Change the location of checkpatch.rst (which we now import)
- 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

And note that we now also include the spdxcheck.py tool that
checkpatch.pl supports calling out to, and include upstream's
checkpatch.rst in our develop section of the documentation.

Signed-off-by: Tom Rini <trini@konsulko.com>