iommu/io-pgtable: Rename iommu_gather_ops to iommu_flush_ops

In preparation for TLB flush gathering in the IOMMU API, rename the
iommu_gather_ops structure in io-pgtable to iommu_flush_ops, which
better describes its purpose and avoids the potential for confusion
between different levels of the API.

$ find linux/ -type f -name '*.[ch]' | xargs sed -i 's/gather_ops/flush_ops/g'

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