drm/tegra: Add falcon helper library

Add a set of falcon helper routines for use by the tegradrm client drivers
of the various falcon-based engines.

The falcon is a microcontroller that acts as a frontend for the rest of a
particular Tegra engine.  In order to properly utilize these engines, the
frontend must be booted before pushing any commands.

Based on work by Andrew Chew <achew@nvidia.com>

Signed-off-by: Andrew Chew <achew@nvidia.com>
Signed-off-by: Arto Merilainen <amerilainen@nvidia.com>
Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>