ARM: KVM: Fix MPIDR computing to support virtual clusters

In order to be able to support more than 4 A7 or A15 CPUs,
we need to fix the MPIDR computing to reflect the fact that
both A15 and A7 can only exist in clusters of at most 4 CPUs.

Fix the MPIDR computing to allow virtual clusters to be exposed
to the guest.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>