iommu: Allow passing custom allocators to pgtable drivers

This will be useful for GPU drivers who want to keep page tables in a
pool so they can:

- keep freed page tables in a free pool and speed-up upcoming page
  table allocations
- batch page table allocation instead of allocating one page at a time
- pre-reserve pages for page tables needed for map/unmap operations,
  to ensure map/unmap operations don't try to allocate memory in paths
  they're allowed to block or fail

It might also be valuable for other aspects of GPU and similar
use-cases, like fine-grained memory accounting and resource limiting.

We will extend the Arm LPAE format to support custom allocators in a
separate commit.

Signed-off-by: Boris Brezillon <[email protected]>
Reviewed-by: Steven Price <[email protected]>
Reviewed-by: Robin Murphy <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Joerg Roedel <[email protected]>

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