dt-bindings: devfreq: tegra30-actmon: Add cooling-cells

The ACTMON watches activity of memory clients. Decisions about a minimum
required frequency are made based on the info from ACTMON. We can use
ACTMON as a thermal cooling device by limiting the required frequency.
Document new cooling-cells property of NVIDIA Tegra ACTMON hardware unit.

Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>