SYDNEY (Reuters) -Singapore Telecommunications (SingTel) is looking to sell a significant stake in its wholly owned Australian unit Optus to Canadian private equity giant Brookfield, a source with direct knowledge of the matter said on Wednesday. Southeast Asia's largest telco operator is in advanced talks with Brookfield over the stake sale, the source said, declining to be named as the information is not public yet. Earlier on Wednesday, SingTel dismissed a report by the Australian Financial Review (AFR) that it was in advanced talks to sell Optus in a deal that could value Australia&...