hw/arm/smmuv3: Fix IIDR offset

The SMMU IIDR register is at 0x018 offset.

Fixes: 10a83cb9 ("hw/arm/smmuv3: Skeleton")
Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20200728150815.11446-9-eric.auger@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>