iommu/arm-smmu: Fix the index calculation of strtab

The element size of cfg->strtab is just one DWORD, so we should use a
multiply operation instead of a shift when calculating the level 1
index.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>