sched: WALT: fix window mis-alignment

The initial window start needs to be close to ktime ns = 0 to be
aligned with scheduler tick.

Change-Id: Ia91f74efce2f910106622a054a6fcd507e763ca5
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>
(cherry picked from commit 0caf1df0c520325c9352149795266709e989b222)
Signed-off-by: Quentin Perret <quentin.perret@arm.com>