MAINTAINERS: Update the DMA Rust entry

Update the DMA Rust maintainers entry in the following two aspects:

  (1) Change Abdiel's entry to 'Reviewer'.

  (2) Take patches through the driver-core tree.

Abdiel won't do any more maintainer work on the DMA (or scatterlist)
infrastructure, but he'd like to be kept in the loop, hence change is
entry to 'R:'.

Analogous to [1], the DMA (and scatterlist) helpers are closely coupled
with the core device infrastructure and the device lifecycle, hence take
patches through the driver-core tree by default.

Cc: Abdiel Janulgue <abdiel.janulgue@gmail.com>
Link: https://lore.kernel.org/r/20250725202840.2251768-1-ojeda@kernel.org

 [1]
Acked-by: Abdiel Janulgue <abdiel.janulgue@gmail.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>