iommu/io-pgtable: Introduce tlb_flush_walk() and tlb_flush_leaf()

In preparation for deferring TLB flushes to iommu_tlb_sync(), introduce
two new synchronous invalidation helpers to the io-pgtable API, which
allow the unmap() code to force invalidation in cases where it cannot be
deferred (e.g. when replacing a table with a block or when TLBI_ON_MAP
is set).

Signed-off-by: Will Deacon <will@kernel.org>