FROMLIST: iommu/arm-smmu: Allow building as a module

By conditionally dropping support for the legacy binding and exporting
the newly introduced 'arm_smmu_impl_init()' function we can allow the
ARM SMMU driver to be built as a module.

Signed-off-by: Will Deacon <will@kernel.org>
Bug: 140290589
Link: https://lore.kernel.org/lkml/20191121114918.2293-14-will@kernel.org/


Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: Id42da5829cb180966a0d051ec636be0c9f90efed