KVM: arm64: Introduce two cache maintenance callbacks

To prepare for performing CMOs for guest stage-2 in the fault handlers
in pgtable.c, here introduce two cache maintenance callbacks in struct
kvm_pgtable_mm_ops. We also adjust the comment alignment for the
existing part but make no real content change at all.

Reviewed-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
[maz: fixed up comments and renamed callbacks]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210617105824.31752-2-wangyanan55@huawei.com