rust: dma: add as_slice/write functions for CoherentAllocation

rust: dma: add as_slice/write functions for CoherentAllocation

Add unsafe accessors for the region for reading or writing large
blocks of data.

Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Abdiel Janulgue <abdiel.janulgue@gmail.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://lore.kernel.org/r/20250602085444.1925053-4-abdiel.janulgue@gmail.com
[ Fix line length and slightly reword safety comment in doc-test of
  CoherentAllocation::write(); fix formatting issue. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>