sched: deadline: WALT: account cumulative runnable avg

Account cumulative runnable average for WALT CPU utilization accounting.

Change-Id: I56934894e626dec183740eeaf89a57d2ef638143
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>
(cherry picked from commit 26b37261ea25c6928c6a69e77a8f7d39ee3267c9)
Signed-off-by: Quentin Perret <quentin.perret@arm.com>