change to two clock pins to drive mics

clock loading was an issue at 1.5 MHz with 13 lines. rigged up a second
board and split mics between the two. Ideal would have a new board with
clock fanout buffers.
implement buffers for future beamforming

107 samples at 48khz needed to beamform at 90 deg. for our array.
individual 128-sample buffers for each mic.