How to set DB log only to log CRON TYPE and SEVERITY NOTICE message?
The Drupal error handler _drupal_error_handler_real() utilized PHP's error_reporting() to determine what gets logged to the DB. By setting the PHP error_reporting to E_ERROR it will only PHP errors. 

This PHP setting can be set in many places: the system php.ini, a php.ini/.user.ini instance in webroot, .htaccess in Apache. The Drupal centric way to do this, is to use ini_set() function in settings.php:

ini_set('error_reporting', E_ERROR);


See PHPs error constant page for other values to set error_reporting.