Adding imx6_add_armpmu to the marsboard init

DS-5/gator is complaining about the PMU counters seeing all zeros. Looking at this page [ https://community.freescale.com/thread/302685 ] I believe it has something to do with the missing "imx6_add_armpmu" function that is in the other imx6 board files (which also seem to have imx6q_add_perfmon functions too). I am adding these in to see if it makes DS-5/gator happy.