iommu/arm-smmu: Convert to domain_alloc_paging()

Now that the BLOCKED and IDENTITY behaviors are managed with their own
domains change to the domain_alloc_paging() op.

The check for using_legacy_binding is now redundant,
arm_smmu_def_domain_type() always returns IOMMU_DOMAIN_IDENTITY for this
mode, so the core code will never attempt to create a DMA domain in the
first place.

Since commit a4fdd976

 ("iommu: Use flush queue capability") the core
code only passes in IDENTITY/BLOCKED/UNMANAGED/DMA domain types. It will
not pass in IDENTITY or BLOCKED if the global statics exist, so the test
for DMA is also redundant now too.

Call arm_smmu_init_domain_context() early if a dev is available.

Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/5-v2-c86cc8c2230e+160bb-smmu_newapi_jgg@nvidia.com
[will: Simplify arm_smmu_domain_alloc_paging() since 'cfg' cannot be NULL]
Signed-off-by: Will Deacon <will@kernel.org>