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>