gpu: nova-core: move GSP boot code to its own module

gpu: nova-core: move GSP boot code to its own module

Right now the GSP boot code is very incomplete and limited to running
FRTS, so having it in `Gpu::new` is not a big constraint.

However, this will change as we add more steps of the GSP boot process,
and not all GPU families follow the same procedure, so having these
steps in a dedicated method is the logical construct.

There is also the fact the GSP will require its own runtime data, and
while it won't immediately need to be pinned, we want to be ready for
the time where it will - most likely when it starts using mutexes.

Thus, add an empty `Gsp` type that is pinned inside `Gpu` and
initialized using a pin initializer. This sets the constraint we need to
observe from the start, and could spare us some costly refactoring down
the road.

Then, move the code related to GSP boot to the `gsp::boot` module, as
part of the `Gsp` implementation.

Doing so allows us to make `Gpu::new` return a fallible `impl PinInit`
instead of a `Result.` This is more idiomatic when working with pinned
objects, and sets up the pinned initialization pattern we want to
preserve as the code grows more complex.

Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250913-nova_firmware-v6-2-9007079548b0@nvidia.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>