of: Add binding for NVIDIA Tegra ACTMON node

This block gathers statistics about various counters and can be configured to
fire interrupts when thresholds are crossed.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
[treding@nvidia.com: rename document, minor cleanups]
Signed-off-by: Thierry Reding <treding@nvidia.com>