dt-bindings: gpu: mali-utgard: Add the #cooling-cells property

The GPU can be one of the big heat sources on a SoC. Allow the
"#cooling-cells" property to be specified for ARM Mali Utgard GPUs so
the GPU clock speeds (and voltages) can be reduced to prevent a SoC from
overheating.

Reviewed-by: Qiang Yu <yuq825@gmail.com>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Rob Herring <robh@kernel.org>