gpu: nova-core: add DMA object struct

Since we will need to allocate lots of distinct memory chunks to be
shared between GPU and CPU, introduce a type dedicated to that. It is a
light wrapper around CoherentAllocation.

Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://lore.kernel.org/r/20250619-nova-frts-v6-13-ecf41ef99252@nvidia.com


Signed-off-by: Danilo Krummrich <dakr@kernel.org>