rust: iommu: add io_pgtable abstraction

rust: iommu: add io_pgtable abstraction

This will be used by the Tyr driver to create and modify the page table
of each address space on the GPU. Each time a mapping gets created or
removed by userspace, Tyr will call into GPUVM, which will figure out
which calls to map_pages and unmap_pages are required to map the data in
question in the page table so that the GPU may access those pages when
using that address space.

The Rust type wraps the struct using a raw pointer rather than the usual
Opaque+ARef approach because Opaque+ARef requires the target type to be
refcounted.

Signed-off-by: Asahi Lina <lina+kernel@asahilina.net>
Acked-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Tested-by: Deborah Brouwer <deborah.brouwer@collabora.com>
Co-developed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
[joro: Fixed up Rust import style]
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>