hps_accel: reduce clock on Arty to 80MHz
hps_accel: reduce clock on Arty to 80MHz

Vivado cannot meet timing currently at 100MHz. Reducing to 80MHz gets it
passing again.

Signed-off-by: Dan Callaghan <c6268a61d01f53ee8e097a439feb27b30b61efda@google.com>

google/CFU-Playground,google/CFU-Playground,google/CFU-Playground,google/CFU-Playground