iommu/io-pgtable-arm: Support all Mali configurations

In principle, Midgard GPUs supporting smaller VA sizes should only
require 3-level pagetables, since level 0 only resolves bits 48:40 of
the address. However, the kbase driver does not appear to have any
notion of a variable start level, and empirically T720 and T820 rapidly
blow up with translation faults unless given a full 4-level table,
despite only supporting a 33-bit VA size.

The 'real' IAS value is still valuable in terms of validating addresses
on map/unmap, so tweak the allocator to allow smaller values while still
forcing the resultant tables to the full 4 levels. As far as I can test,
this should make all known Midgard variants happy.

Fixes: d08d42de ("iommu: io-pgtable: Add ARM Mali midgard MMU page table format")
Tested-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>