Category and level both use an enum for their ID values. Add functions to
convert these IDs to strings and vice versa. This will allow the log to
output the strings instead of the (inscrutable) values.

At the same time, add a new 'driver-model' category, to cover core
driver-model functions and fix an incorrect value for LOGL_MAX.

Tests will be added with the new 'log' subcommands.

Signed-off-by: Simon Glass <sjg@chromium.org>
(Updated to correct clang warnings)