sched: cpufreq: Adds a field cpu_power in the task_struct

cpu_power has been added to keep track of amount of power each task is
consuming. cpu_power is updated whenever stime and utime are updated for
a task. power is computed by taking into account the frequency at which
the current core was running and the current for cpu actively
running at hat frequency.

Change-Id: Ic535941e7b339aab5cae9081a34049daeb44b248
Signed-off-by: Ruchi Kandoi <kandoiruchi@google.com>

Original commit: https://github.com/dianlujitao/CAF_kernel_msm-3.10/commit/85a6bd2bc4c903df43186e6f41209746aa6fdf05