dt-bindings: gpu: add #cooling-cells property to the ARM Mali Midgard GPU binding

The GPU can be used as a thermal cooling device, add an optional
'#cooling-cells' property.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20190516172510.181473-1-mka@chromium.org