sched: Add scaled task load statistics

Scheduler guided frequency selection as well as task placement on
heterogeneous systems require scaled task load statistics. This patch
adds a 'runnable_avg_sum_scaled' metric per task that is a scaled
derivative of 'runnable_avg_sum'. Load is scaled in reference to
"best" cpu, i.e one with best possible max_freq

Change-Id: Ie8ae450d0b02753e9927fb769aee734c6d33190f
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>