arm64: dts: rockchip: Fix multiple thermal zones conflict in rk3399.dtsi

Currently the common thermal zones definitions for the rk3399 assumes
multiple thermal zones are supported by the governors. This is not the
case and each thermal zone has its own governor instance acting
individually without collaboration with other governors.

As the cooling device for the CPU and the GPU thermal zones is the
same, each governors take different decisions for the same cooling
device leading to conflicting instructions and an erratic behavior.

As the cooling-maps is about to become an optional property, let's
remove the cpu cooling device map from the GPU thermal zone.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>