iommu/arm-smmu: allow 42-bit stage-1 output size with 64K pages

The output size of stage-1 is currently limited by the input size of
stage-2, which is further limited by VA_BITS since we make use of the
standard pgd_alloc functions for creating page tables.

This patch ensures that we use VA_BITS instead of a hardcoded '39'
for the stage-1 output size limit.

Signed-off-by: Will Deacon <will.deacon@arm.com>