iommu/arm-smmu: don't bother truncating the s1 output size to VA_BITS

In order for nested translation to work correctly, we need to ensure
that the maximum output address size from stage-1 is <= the maximum
supported input address size to stage-2. The latter is currently defined
by VA_BITS, since we make use of the CPU page table functions for
allocating out tables and so the driver currently enforces this
restriction by truncating the stage-1 output size during probe.

In reality, this doesn't make a lot of sense; the guest OS is responsible
for managing the stage-1 page tables, so we actually just need to ensure
that the ID registers of the virtual SMMU interface only advertise the
supported stage-2 input size.

This patch fixes the problem by treating the stage-1 and stage-2 input
address sizes separately.

Reported-by: Tirumalesh Chalamarla <tchalamarla@cavium.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>