docs: Debugging390.txt: convert table to ascii artwork

The first bit/value table inside the document is very
hard to read and won't fit ReST format. Also, some columns aren't
properly aligned.

Convert it to a nice ascii artwork table with makes it easier to
read as plain text and is compatible with ReST format parser
on Sphinx.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>