lpm-levels: Fix irqsoff tracing with {stop,start}_critical_timings()

The lpm-levels code uses spinlocks and preemption disabling
primitives up until the time that we call cpu_suspend() or wfi.
Add some stop_critical_timings()/start_critical_timings() calls
so that the irqsoff tracer disables itself across periods of long
idle and suspend activity. Otherwise, we will only see the idle
path in the trace file when using the irqsoff tracer.

Change-Id: I3535b88d99cc221b33806adaacb280f0cb357e62
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>