STANBUL, April 2 (Reuters) - Police in five European states detained 63 people in raids against a Turkish far-left militant group, an official said on Friday, in a security sweep Turkish media reports said was linked to a NATO summit in June. Istanbul will host leaders of the Atlantic alliance, including U.S. President George W. Bush, on June 28-29 and some newspapers said authorities wanted to rein in militant groups before then. Police and interior ministry officials were not immediately available to comment on the reports. The group targeted in Thursday's raids, the Revolutionary People's Liberation Party-Front (DHKP-C), is known...