Add LayoutExternalPort for interface

def addPort(n: LNode, intf: Interface):
    """
    Add LayoutExternalPort for interface
    """
    d = PortTypeFromDir(intf._direction)
    ext_p = LayoutExternalPort(
        n, name=intf._name, direction=d, node2lnode=n._node2lnode)
    ext_p.originObj = originObjOfPort(intf)
    n.children.append(ext_p)
    addPortToLNode(ext_p, intf, reverseDirection=True)
    return ext_p