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>

doc/driver-model/debugging.rst

@@ -58,5 +58,5 @@ If you are using of-platdata (e.g. CONFIG_SPL_OF_PLATDATA), check that the
 driver name is the same as the first compatible string in the device tree (with
 invalid-variable characters converted to underscore).
 
-If you are really stuck, #define DEBUG at the top of lists.c should show you
-what is going on.
+If you are really stuck, putting '#define LOG_DEBUG' at the top of
+drivers/core/lists.c should show you what is going on.