sched: create a new field with available capacity

Date	Fri, 18 Oct 2013 13:52:19 +0200

This new field power_available reflects the available capacity of a CPU
unlike the cpu_power which reflects the current capacity.

Signed-off-by: Vincent Guittot <vincent.guittot at linaro.org>
Signed-off-by: flar2 <asegaert@gmail.com>