Configure it by adding an entry to config/authsources.php such as this:
'authtfaga' => array(
'authtfaga:authtfaga',
'db.dsn' => 'mysql:host=db.example.com;port=3306;dbname=idpauthtfaga',
'db.username' => 'simplesaml',
'db.password' => 'bigsecret',
'mainAuthSource' => 'ldap',
'uidField' => 'uid',
'totpIssuer' => 'dev_aai_teszt_IdP'
),