sched: HMP: Additional trace points for debugging HMP behaviour

sched: HMP: Additional trace points for debugging HMP behaviour

1. Replace magic numbers in code for migration trace.
   Trace points still emit a number as force=<n> field:
     force=0 : wakeup migration
     force=1 : forced migration
     force=2 : offload migration
     force=3 : idle pull migration

2. Add trace to expose offload decision-making.
   Also adds tracing rq->nr_running so that you can
   look back to see what state the RQ was in at the time.

Signed-off-by: Chris Redpath <chris.redpath@arm.com>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Jon Medhurst <tixy@linaro.org>