drm/nouveau/secboot: reorganize into more files

Split the act of building the ACR blob from firmware files from the rest
of the (chip-dependent) secure boot logic. ACR logic is moved into
acr_rxxx.c files, where rxxx corresponds to the compatible release of
the NVIDIA driver. At the moment r352 and r361 are supported since
firmwares have been released for these versions. Some abstractions are
added on top of r352 so r361 can easily be implemented on top of it by
just overriding a few hooks.

This split makes it possible and easy to reuse the same ACR version on
different chips. It also hopefully makes the code much more readable as
the different secure boot logics are separated. As more chips and
firmware versions will be supported, this is a necessity to not get lost
in code that is already quite complex.

This is a big commit, but it essentially moves things around (and split
the nvkm_secboot structure into two, nvkm_secboot and nvkm_acr). Code
semantics should not be affected.

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