ARM: dts: rockchip: Use GPU as cooling device for the GPU thermal zone of the rk3288

Currently the CPUs are used as cooling devices of the rk3288 GPU
thermal zone. The CPUs are also configured as cooling devices in the
CPU thermal zone, which indirectly helps with cooling the GPU thermal
zone, since the CPU and GPU temperatures are correlated on the rk3288.

Configure the ARM Mali Midgard GPU as cooling device for the GPU
thermal zone instead of the CPUs.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>