ARM: tegra: Add cooling cells to ACTMON device-tree node

The ACTMON module monitors activity of memory clients and decisions
about a minimum required memory frequency are made based on info from
ACTMON. Add cooling cells to ACTMON device-tree node in order to turn
it into a cooling device that will throttle memory freq on overheat.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>