drm/nouveau/secboot: add LS firmware post-run hooks

Add the ability for LS firmwares to declare a post-run hook that is
invoked right after the HS firmware is executed. This allows them to
e.g. write some initialization data into the falcon's DMEM.

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