Update fast mixer statistics

Compute statistics on fast mixer elapsed time and CPU load per cycle using a
simple moving average rather than by fixed blocks.  This has a couple advantages:
 - remove burstiness of CPU usage due to former floating-point calculations in fast mixer
 - gives us flexibility in how to report (e.g. could report over just the last 1 second)

Disadvantage is increased RAM, and since the samples are not updated
atomically, it is possible to have an error in the statistics.  This
should not be much of an issue due to the relatively large number of samples.

Also add CPU raw ns and adjusted MHz statistics.

Change-Id: Iaa2cd13f18250c3162aff40409b3694b769d9505