KVM: arm64: Introduce mm_ops member for structure stage2_attr_data

Also add a mm_ops member for structure stage2_attr_data, since we
will move I-cache maintenance for guest stage-2 to the permission
path and as a result will need mm_ops for some callbacks.

Reviewed-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210617105824.31752-3-wangyanan55@huawei.com