samples: rust: dma: add sample code for SGTable

samples: rust: dma: add sample code for SGTable

Add sample code for allocating and mapping a scatter-gather table
(`SGTable`).

Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Co-developed-by: Abdiel Janulgue <abdiel.janulgue@gmail.com>
Signed-off-by: Abdiel Janulgue <abdiel.janulgue@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250828133323.53311-5-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>