arm64: dts: rockchip: Add idle cooling devices to rk3399

The thermal framework accepts now the cpu idle cooling device as an
alternative when the cpufreq cooling device fails.

Add the node in the DT so the cooling devices will be present and the
platforms can extend the thermal zone definition to add them.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20211001161728.1729664-1-daniel.lezcano@linaro.org


Signed-off-by: Heiko Stuebner <heiko@sntech.de>