KVM: arm/arm64: Bump VGIC_V3_MAX_CPUS to 512

Let's raise the number of supported vcpus along with
vgic v3 now that HW is looming with more physical CPUs.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Acked-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>