drm/nouveau/secboot: add low-secure firmware hooks

Secure firmwares provided by NVIDIA will follow the same overall
principle, but may slightly differ in format, or not use the same
bootloader descriptor even on the same chip. In order to handle
this as gracefully as possible, turn the LS firmware functions into
hooks that can be overloaded as needed.

The current hooks cover the external firmware loading as well as the
bootloader descriptor generation.

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