power: Enable retention only on cpu0

qualcomm only uses all-core retention on 8974, so lets not be greedy

Change-Id: Ie27805a37b9b3b8a15e501e405690aac87ca6477
Revert "Enable new power modes"

This reverts commit cd147da5eeb9f5e2620cab410a65b5d4ee67c571.