dt-bindings: gpu: Add Mali Utgard bindings

The ARM Mali Utgard GPU family is embedded into a number of SoCs from
Allwinner, Amlogic, Mediatek or Rockchip.

Add a binding for the GPU of that family.

Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>