drm/nouveau/gsp: add gpu hal stubs

With GSP-RM handling the majority of the HW programming, NVKM's usual
HALs are more elaborate than necessary, resulting in a fair amount of
duplicated boilerplate.

Adds 'nvkm_rm_gpu' which serves to provide GPU-specific constants and
functions in a more streamlined manner.

This is initially used in subsequent commits to store engine class IDs,
and replace the per-engine/engobj boilerplate with common code for all
GSP-RM supported engines - and is further extended when adding GH100,
GB10x and GB20x support.

Signed-off-by: Ben Skeggs <bskeggs@nvidia.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Timur Tabi <ttabi@nvidia.com>
Tested-by: Timur Tabi <ttabi@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>