arm64: Add AT operation encodings

Add the encodings for the AT operation that are usable from NS.

Reviewed-by: Eric Auger <eric.auger@redhat.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Miguel Luis <miguel.luis@oracle.com>
Reviewed-by: Zenghui Yu <yuzenghui@huawei.com>
Reviewed-by: Jing Zhang <jingzhangos@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230815183903.2735724-6-maz@kernel.org