ARM: dts: rockchip: Add #cooling-cells entry for rk3288 GPU

The Mali GPU of the rk3288 can be used as cooling device, add
a #cooling-cells entry for it.

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