HONG KONG, June 20 (Reuters) - Dealmaking in Southeast Asia and private equity transactions should help lift Asian mergers and acquisitions (M&A) activity in the second half of the year, bankers said, after deals volume in the first half slumped to the lowest in a decade. Total Asia M&A value for January through June dropped 41% year on year to reach $362 billion, the lowest since 2013, preliminary data from Refinitiv showed. The decline was roughly in line with the rest of the world, as higher interest rates, volatile markets and geopolitical tensions weighed on dealmaking globally, causing a number...