By David Lawder WASHINGTON (Reuters) - President Donald Trump swore in former Goldman Sachs banker and Hollywood financier Steven Mnuchin as Treasury secretary on Monday, putting him to work on tax reform, financial de-regulation and economic diplomacy efforts. The U.S. Senate voted to confirm Mnuchin 53-47, with all but one Democrat opposing him over his handling of thousands of foreclosures as head of OneWest Bank after the 2007-2009 housing collapse. At a White House swearing-in ceremony, Trump said Mnuchin would be a "great champion" for U.S. citizens.