ARM: KVM: Remove pointless void pointer cast

No need to cast the void pointer returned by kmalloc() in
arch/arm/kvm/mmu.c::kvm_alloc_stage2_pgd().

Signed-off-by: Firo Yang <firogm@gmail.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>