Silicon Valley venture capital giant New Enterprise Associates, known as NEA, is likely to open a Boston area office within the next six months. That’s according to General Partner David Mott, leader of the venture firm’s health care investing and former CEO of drug maker Medimmune, which was bought by AstraZeneca plc (NYSE: AZN) in 2007. Mott is based in Washington D.C. but says he spends so much of his time in Boston, that it only makes sense to put out a shingle here. “Boston is winning the…