At present it is possible to '#define DEBUG' at the top of a file which
causes all debug() statements in that file to become active. There is
currently no equivalent with logging, but this is a useful function.

Add a LOG_DEBUG define along with documentation.
Signed-off-by: NSimon Glass <sjg@chromium.org>
Reviewed-by: NBin Meng <bmeng.cn@gmail.com>