All of this code is going to be moved out of `Gpu::new()` eventually
(i.e. the follow-up patchset), but we are still figuring out where it
will eventually land. We will need some other entity to manage the GSP
boot (GspBooter?), and I am still learning which parts are common to all
GPU families and which ones should be a HAL. So for now I'd rather keep
it here, modulo the part that can be moved into `FwsecFirmware`.