New event for minstret counter.

Created by: silabs-oysteink

Instead of using wb_valid which factors in data_rvalid_i, minstret is counted for the first cycle of a valid instruction in WB. For LSU instructions with stalls on rvalid, this means we count before the instruction retires. RVFI is altered to detect this and report correct values for the retired instruction.

Signed-off-by: Oystein Knauserud Oystein.Knauserud@silabs.com