drm/nouveau/secboot: support for loading LS PMU firmware

Allow secboot to load a LS PMU firmware. LS PMU is one instance of
firmwares based on the message queue mechanism, which is also used for
other firmwares like SEC, so name its source file accordingly.

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