dev-arm: FVPBasePwrCtrl, fix vector resizing

dev-arm: FVPBasePwrCtrl, fix vector resizing

(1) ThreadContexts are registered into System in BaseCPU::init.
(2) FVPBasePwrCtrl state is resized based on registered ThreadContexts
in FVPBasePwrCtrl::init.

FVPBasePwrCtrl::init may be called before BaseCPU::init based on the
model names alphabetical order, leading to segmentation faults.
To fix this, (2) is now carried out in FVPBasePwrCtrl::startup.

Change-Id: Ica6c5b7448da556d61aee53f8777a709fcad2212
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35075
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>