drm/nouveau/secboot: base support for PMU falcon

Adapt secboot's behavior if a PMU firmware is present, in particular
the way LS falcons are reset. Without PMU firmware, secboot needs to be
performed again from scratch so all LS falcons are reset. With PMU
firmware, we can ask the PMU's ACR unit to reset a specific falcon
through a PMU message.

As we must preserve the old behavior to avoid breaking user-space, add a
few conditionals to the way falcons are reset.

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