dt-bindings: gpu: mali-utgard: add mediatek, mt7623-mali compatible

The MediaTek MT7623 SoC contains a Mali-450, so add a compatible for it
and define its own vendor-specific properties.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/efeadefe3895bcadf1d2e9847b82206dd8c7ec35.1563867856.git.ryder.lee@mediatek.com


[mb: move to yaml file]
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>