rust: dma: add from-slice constructors for Coherent and CoherentBox

rust: dma: add from-slice constructors for Coherent and CoherentBox

A very common pattern is to create a block of coherent memory with the
content of an already-existing slice of bytes (e.g. a loaded firmware
blob).

`CoherentBox` makes this easier, but still implies a potentially
panicking operation with `copy_from_slice` that requires a `PANIC`
comment.

Add `from_slice_with_attrs` and `from_slice` methods to both `Coherent`
and `CoherentBox` to turn this into a trivial one-step operation.

Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20260327-b4-nova-dma-removal-v2-1-616e1d0b5cb3@nvidia.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>