ARM: tegra: nexus7: Enable memory frequency thermal throttling using ACTMON

The ACTMON module monitors activity of memory clients and then devfreq
driver makes decisions about a required memory frequency based on info
from ACTMON. Add ACTMON device to the thermal zone of Nexus 7 in order
to use it as a cooling device which throttles memory freq on overheat.

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