arm/arm64: gicv3: support up to 8 redistributor regions

We need to support at least two redistributor regions in order to
support more than 123 vcpus (we select 8 because that should be
plenty). Also bump NR_CPUS to 512, since that's what KVM currently
supports.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Tested-by: Christoffer Dall <christoffer.dall@arm.com>
Reviewed-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>