The array of data you can pass to the \Magento\Eav\Setup\EavSetup::addAttribute() method is as follows:
Some of the attribute keys are remapped using the Magento\Eav\Model\Entity\Setup\PropertyMapper class. We indicate what it's mapped to in [] brackets
$data = [
'type' => 'varchar',
// (string)[eav_attribute][backend_type]: database storage type (varchar|int|text|decimal)
'backend' => NULL,
// (string)[eav_attribute][backend_model]: class name used to retrieve/save the attribute data to the db
'frontend' => NULL,