dt-bindings: gpu: drop wrong compatible from midgard binding example

The binding rightfully only specifies compatibles for actual implementations
and not for the whole family. In the example an arm,mali-midgard slipped
through from downstream devicetrees though. So drop that to not confuse
people reading (or copying) that example.
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/gpu/arm,mali-midgard.txt

@@ -40,7 +40,7 @@ Optional properties:
 Example for a Mali-T760:
 
 gpu@ffa30000 {
-	compatible = "rockchip,rk3288-mali", "arm,mali-t760", "arm,mali-midgard";
+	compatible = "rockchip,rk3288-mali", "arm,mali-t760";
 	reg = <0xffa30000 0x10000>;
 	interrupts = <GIC_SPI 6 IRQ_TYPE_LEVEL_HIGH>,
 		     <GIC_SPI 7 IRQ_TYPE_LEVEL_HIGH>,