iommu/io-pgtable-arm: Fix pgtable allocation in selftest

Commit 4b123757

 ("iommu/io-pgtable-arm: Make allocations
NUMA-aware") added a NUMA hint to page table allocation, but the pgtable
selftest doesn't provide an SMMU device parameter. Since dev_to_node
doesn't accept a NULL argument, add a special case for selftest.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>