iommu/arm-smmu-v3: Remove the unused fields for PREFETCH_CONFIG command

Per SMMUv3 spec, there is no Size and Addr field in the PREFETCH_CONFIG
command and they're not used by the driver. Remove them.

We can add them back if we're going to use PREFETCH_ADDR in the future.

Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Link: https://lore.kernel.org/r/20210407084448.1838-1-yuzenghui@huawei.com
Signed-off-by: Will Deacon <will@kernel.org>