of: Add bindings of hw throttle for Tegra soctherm

Add HW throttle configuration sub-node for soctherm, which
is used to describe the throttle event, and worked as a
cooling device. The "hot" type trip in thermal zone can
be bound to this cooling device, and trigger the throttle
function.

Signed-off-by: Wei Ni <wni@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>