arm64: dts: rockchip: Add GPU cooling device for RK3399

As for RK3288, now that we have a binding for the GPU we can
hook up the missing cooling device for the thermal zone.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/cb905e17526d846d6d35fb86fbd3c8ba4af4cdaf.1574974673.git.robin.murphy@arm.com


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