ARM: KVM: Move CP15 array into the CPU context structure

Continuing our rework of the CPU context, we now move the CP15
array into the CPU context structure. As this causes quite a bit
of churn, we introduce the vcpu_cp15() macro that abstract the
location of the actual array. This will probably help next time
we have to revisit that code.

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