sched: walt: add the update_task_ravg_mini trace event

The update_task_ravg trace event takes up quite a lot of space
both in the binary ftrace format as well as when the final human
readable trace is generated. Add an event that reduces this
footprint while still retaining the most valuable information.

Change-Id: I918866ab4a01fa8c2a0c7bde51e336e4c8762b12
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>