sched/fair: Change PELT half-life to 8ms

PELT's common ratio is based on a 32ms half life. Change this to 8ms.
Adapted from Patrick Bellasi's (patrick.bellasi@arm.com) patch, available
here: https://patchwork.kernel.org/patch/10331795/

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Change-Id: I67ad8c80811569b7eaea2ca6e76840c6f20f250e
Signed-off-by: Jonathan Avila <avilaj@codeaurora.org>
Signed-off-by: Sai Harshini Nimmala <snimmala@codeaurora.org>