iommu/io-pgtable-arm: Fix stage-2 concatenation with 16K

iommu/io-pgtable-arm: Fix stage-2 concatenation with 16K

At the moment, io-pgtable-arm uses concatenation only if it is
possible at level 0, which misses a case where concatenation is
mandatory at level 1 according to R_SRKBC in Arm spec DDI0487 K.a.

Also, that means concatenation can be used when not mandated,
contradicting the comment on the code. However, these cases can only
happen if the SMMUv3 driver is changed to use ias != oas for stage-2.

This patch re-writes the code to use concatenation only if mandatory,
fixing the missing case for level-1 and granule 16K with PA = 40 bits.

Signed-off-by: Mostafa Saleh <smostafa@google.com>
Link: https://lore.kernel.org/r/20241202140604.422235-2-smostafa@google.com
Signed-off-by: Will Deacon <will@kernel.org>