dev-arm: Move GICv3 (Re)Ditributor address in Realview.py

dev-arm: Move GICv3 (Re)Ditributor address in Realview.py

Base addresses for GICv3's Distributor and Redistributors are
implementation defined: they depend on the platform rather than the
model. This patch is then moving dist_addr and redist_addr
initialization in Realview.py

Change-Id: I1246df500262f4d3d5a38e62d0240945f90941ee
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18393
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>