iommu/io-pgtable-arm-selftests: Modularize the test

iommu/io-pgtable-arm-selftests: Modularize the test

Remove the __init constraint, as the test will be converted to KUnit,
it can run on-demand after later.

Also, as KUnit can be a module, make this test modular.

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Pranjal Shrivastava <praan@google.com>
Signed-off-by: Mostafa Saleh <smostafa@google.com>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>