system-arm: GICv2/GICv3 have different Distributor addresses

system-arm: GICv2/GICv3 have different Distributor addresses

https://gem5-review.googlesource.com/c/public/gem5/+/22823 didn't
take into consideration that GICv3's Distributor is placed at a
different address than GICv2's one.
This is reflected by the value in VExpress_GEM5_V2 and in the
FDT in system/arm/dt/platforms/vexpress_gem5_v2_base.dtsi

Change-Id: Ie7661d4e9d3db0c5fe9eb9cea3a24a5e7c266676
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Adrian Herrera <adrian.herrera@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23953
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>