gpu: nova-core: expose the offset of each register as a type constant

Although we want to access registers using the provided methods, it is
sometimes needed to use their raw offset, for instance when working with
a register array.

Expose the offset of each register using a type constant to avoid
resorting to hardcoded values.

Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://lore.kernel.org/r/20250619-nova-frts-v6-8-ecf41ef99252@nvidia.com


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