By Amanda Ferguson and Elizabeth Piper BALLYNAHINCH, Northern Ireland (Reuters) -There are still genuine differences between British and European Union negotiators that could take some time to resolve if talks on revising post-Brexit trade rules for Northern Ireland are to succeed, Britain’s foreign minister said on Wednesday. James Cleverly was speaking after meeting political and business leaders in Northern Ireland to discuss difficulties about the trading arrangements after a mellowing in a years-long standoff yielded some progress this week. “Optimism is great, having a spirit of optimism…