doc: dm: debugging: Fix the steps for activating debug

doc: dm: debugging: Fix the steps for activating debug

Following the recommendation of adding '#define DEBUG' at the top
of drivers/core/lists.c does not cause the debug messages to be
shown. Change it to '#define LOG_DEBUG' instead, which actually
makes it work as per doc/README.log.

While at it, provide the full path to lists.c to in order to make
the instructions clearer.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>