KVM: arm/arm64: Comments cleanup in mmu.c

Some comments in virt/kvm/arm/mmu.c are outdated. Update them to
reflect the current state of the code.

Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
[maz: commit message tidy-up]
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>