drm/nouveau/secboot: generate HS BL descriptor in hook

Use the HS hook to completely generate the HS BL descriptor, similarly
to what is done in the LS hook, instead of (arbitrarily) using the
acr_v1 format as an intermediate.

This allows us to make the bootloader descriptor structures private to
each implementation, resulting in a cleaner an more consistent design.

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