samples: rust: add Rust dma test sample driver

Add a simple driver to exercise the basics of the Rust DMA
coherent allocator bindings.

Suggested-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Abdiel Janulgue <abdiel.janulgue@gmail.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250317185345.2608976-4-abdiel.janulgue@gmail.com


[ Renamed Kconfig symbol and moved it up. Migrated to the new
  `authors` key in `module!`. Fixed module name in description
  and typo in commit message. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>