iommu/io-pgtable-arm: Move selftests to a separate file

iommu/io-pgtable-arm: Move selftests to a separate file

Clean up the io-pgtable-arm library by moving the selftests out.
Next the tests will be registered with kunit.

This is useful also to factor out kernel specific code out, so
it can compiled as part of the hypervisor object.

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>