Add machdep.cpu_idle_max_stop_state tunable

Multiple idle and deep sleep states are supported on Power7+ platforms, with
each state having a characteristic latency and effect on system real-time
performance.

Add a tunable to allow a maximum sleep state (maximum theoretical sleep-related
latency) to be set, in order to better support specific latency-sensitive
applications / real time workloads.

Signed-off-by: Timothy Pearson <tpearson@raptorengineering.com>