By Christoph Steitz FRANKFURT (Reuters) -Volkswagen and its top shareholder Porsche SE on Thursday fleshed out details of a possible listing of luxury carmaker Porsche, edging closer to what could become one of the world’s largest stock market debuts. In case of an initial public offering, the share capital of Porsche AG would be equally split into preferred and ordinary shares and up to a quarter of the preferred stock would be placed on the market, Volkswagen said, confirming an earlier Reuters story. This implies a potential placement and free float of up to 12.5% of Porsche AG’s total shar…