arm: Stop serializing ISA values wihch are cached from the system.

arm: Stop serializing ISA values wihch are cached from the system.

These values are not really part of the ISA state and could be
retrieved from the system during execution. Also these values are
already being set in the ISA constructor.

Change-Id: Iea5f9bbb27add4ecebc6391da6c1c1e49e76508f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22543
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>