gpu: nova-core: update and annotate TODO list

gpu: nova-core: update and annotate TODO list

A few new dependencies are required to remove some of the TODO items:

- A way to safely convert from byte slices to types implementing
  `FromBytes`,
- A way to obtain slices and write into a `CoherentAllocation`,
- Several improvements to the `register!()` macro,
- Alignment operations to powers of two, and an equivalent to the C
  `fls`,
- Support for `xa_alloc` in the XAlloc bindings.

Some items have also become obsolete:

- The auxiliary bus abstractions have been implemented and are in use,
- The ELF utilities are not considered for being part of the core kernel
  bindings anymore.
- VBIOS, falcon and GPU timer have been completed.

We now have quite a few TODO entries in the code, so annotate them with
a 4 letter code representing the corresponding task in `todo.rst`. This
allows to easily find which part of the code corresponds to a given
entry (and conversely).

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://lore.kernel.org/r/20250619-nova-frts-v6-24-ecf41ef99252@nvidia.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>