gpu: nova-core: firmware: gsp: use dma::Coherent for level0 table

gpu: nova-core: firmware: gsp: use dma::Coherent for level0 table

Replace the nova-core local `DmaObject` with a `CoherentBox` that can
fulfill the same role.

Since `CoherentBox` is more flexible than `DmaObject`, we can use the
native `u64` type for page table entries instead of messing with bytes.

The `dma` module becomes unused with that change, so remove it as well.

Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20260327-b4-nova-dma-removal-v2-7-616e1d0b5cb3@nvidia.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>