drm/nouveau/falcon: add msgqueue interface

A message queue firmware implements a specific protocol allowing the
host to send "commands" to a falcon, and the falcon to reply using
"messages". This patch implements the common part of this protocol and
defines the interface that the host can use.

Due to the way the firmware is developped internally at NVIDIA (where
kernel driver and firmware evolve in lockstep), firmwares taken at
different points in time can have frustratingly subtle differences that
must be taken into account. This code is architectured to make
implementing such differences as easy as possible.

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