iommu/arm-smmu: fix table flushing during initial allocations

Now that we populate page tables as we traverse them ("iommu/arm-smmu:
fix pud/pmd entry fill sequence"), we need to ensure that we flush out
our zeroed tables after initial allocation, to prevent speculative TLB
fills using bogus data.

This patch adds additional calls to arm_smmu_flush_pgtable during
initial table allocation, and moves the dsb required by coherent table
walkers into the helper.

Cc: <stable@vger.kernel.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>