drm/nouveau/gsp: switch to a simpler GSP-RM header layout

Rather than using OpenRM's directory structure for headers, move to a
layout that's split roughly around RM API boundaries.

Also move the headers from include/nvrm to subdev/gsp/rm/r535/nvrm,
with the rest of the r535-specific code.

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>