system-arm: update armv8 cpu-release-addr

In the cl below, cpu-release-addr is changed to 0x87fffff8.
https://gem5-review.googlesource.com/c/public/gem5/+/35076

By fixing cpu-release-addr, we are able to bring up multi-core platform.

Change-Id: I48bb678f67b677e9fc0136c378407e06ce7a46f4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/42484
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>
2 files changed