sched: deadline: use deadline bandwidth in scale_rt_capacity

Instead of monitoring the exec time of deadline tasks to evaluate the
CPU capacity consumed by deadline scheduler class, we can directly
calculate it thanks to the sum of utilization of deadline tasks on the
CPU.  We can remove deadline tasks from rt_avg metric and directly use
the average bandwidth of deadline scheduler in scale_rt_capacity.

Based in part on a similar patch from Luca Abeni <luca.abeni@unitn.it>.

Change-Id: Ida81c088f21496dec049da9ee39dd17319d357d2
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Steve Muckle <smuckle@linaro.org>
(am from https://patchwork.kernel.org/patch/7804861/

)
Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>