dt-bindings: gpu: mali-utgard: Add Allwinner R40 compatible

Allwinner R40 SoC contains Mali400, so add its specific compatible to
bindings.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20200824150434.951693-2-jernej.skrabec@siol.net