iommu/io-pgtable-arm: Add coverage for different OAS in selftest

iommu/io-pgtable-arm: Add coverage for different OAS in selftest

Run selftests with different OAS values intead of hardcoding it to 48
bits.

We always keep OAS >= IAS to make the config valid for stage-2.

This can be further improved, if we split IAS/OAS configuration for
stage-1 and stage-2 (to use input sizes compatible with VA_BITS as
SMMUv3 does, or IAS > OAS which is valid for stage-1).

However, that adds more complexity, and the current change improves
coverage and makes it possible to test all concatenation cases.

Signed-off-by: Mostafa Saleh <smostafa@google.com>
Link: https://lore.kernel.org/r/20241202140604.422235-3-smostafa@google.com
Signed-off-by: Will Deacon <will@kernel.org>