dt-bindings: gpu: vivante,gc: Add common properties

Add common properties appearing in DTSes (cooling-cells, assigned-clocks
and others) to fix dtbs_check warnings like:

  arch/arm64/boot/dts/freescale/imx8mq-evk.dt.yaml: gpu@38000000:
    '#cooling-cells', 'assigned-clock-parents', 'assigned-clock-rates', 'assigned-clocks' do not match any of the regexes: 'pinctrl-[0-9]+'

Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200904145312.10960-5-krzk@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>