iommu/arm-smmu-v3: Add a type for the STE

Instead of passing a naked __le16 * around to represent a STE wrap it in a
"struct arm_smmu_ste" with an array of the correct size. This makes it
much clearer which functions will comprise the "STE API".

Reviewed-by: Moritz Fischer <mdf@kernel.org>
Reviewed-by: Michael Shavit <mshavit@google.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Tested-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Will Deacon <will@kernel.org>