Dorchester-based developer Rise Together has told the BPDA it will soon file detailed plans for two six-story residential buildings in Mattapan Square, one to replace a repair garage at 1590 Blue Hill Ave., and the other behind it on River Street. The Blue Hill Avenue building would have 65 residential units and a “locally owned and operated family-style sit-down restaurant,” while the River Street building would have 100 residential units and a 44-space parking garage, the developer said in a “letter of intent” filed with the BPDA.