kernel: sched: account for real time utilization

PELT doesn't account for real time task utilization in cpu_util().
As the result a CPU busy running RT task is considered as low
utilization by the scheduler. Fix this by adding real time loading
in to account.

Bug: 147385228
Test: boot to home and run audio test

Change-Id: Ie4412b186608b9a618f0d35cee9a7310db481f7c
Signed-off-by: Kyle Lin <kylelin@google.com>
Signed-off-by: Alexander Winkowski <dereference23@outlook.com>