sched: Make task and CPU load calculations safe from truncation

Load calculations have been modified to accept and return 64 bit values.
Fix up all the places where we make such calculations to store the result
in 64 bit variables. This is necessary to avoid issues caused by
truncation of values.

While at it update scale_task_load() to scale_load_to_cpu(). This is
because the API is used to scale load of both individual tasks as well as
the cumulative load of CPUs. In this sense the name was a misnomer. Also
clean up power_cost() to use max_task_load().

Change-Id: I51e683e1592a5ea3c4e4b2b06d7a7339a49cce9c
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>