iommu/arm-smmu: Move .tlb_sync method to implementation

With the .tlb_sync interface no longer exposed directly to io-pgtable,
strip away the remains of that abstraction layer. Retain the callback
in spirit, though, by transforming it into an implementation override
for the low-level sync routine itself, for which we will have at least
one user.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>