wp_update_post via ajax from frontend

You can stop post revisions using define('WP_POST_REVISIONS', false); in wp-config.php or try below if it works.

function save_page() {
    exit( wp_update_post( array(
        'ID'           => absint( esc_attr( $_POST['postID'] ) ),
        'post_type'    =>'your post type'
        'post_content' => esc_attr( $_POST['content'] )
    ) ) );
}