drm/nouveau/gm20b: add dummy PMU device

Add a dummy PMU device so the PMU falcon is instanciated and can be used
by secure boot.

We could reuse gk20a's implementation here, but it would fight with
secboot over PMU falcon's ownership and secboot will reset the PMU,
preventing it from operating afterwards. Proper handout between secboot
and pmu is coming along with the actual gm20b PMU implementation, so
use this as a temporary solution.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>