Implements hook_field_widget_info_alter().
function entity_reference_field_widget_info_alter(&$info) {
if (isset($info['options_select'])) {
$info['options_select']['field_types'][] = 'entity_reference';
}
if (isset($info['options_buttons'])) {
$info['options_buttons']['field_types'][] = 'entity_reference';
}
}