scripts/checkpatch: add missing spelling.txt

Checkpatch can use two dictionaries for finding typos:
* scripts/spelling.txt
* /usr/share/codespell/dictionary.txt
  which comes with package codespell and is selected by parameter
  --codespell

If the file scripts/spelling.txt is missing, a warning is shown.

Add the spelling.txt file from the Linux kernel 4.13 source.

We can later add our own typo corrections.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Dan Murphy <dmurphy@ti.com>