dt-bindings: mali-bifrost: add dma-coherent

Add optional dma-coherent property to binding doc.

Found by 'make dtbs_check' on arm64/amlogic DT files.

Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210421204833.18523-2-khilman@baylibre.com'
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Documentation/devicetree/bindings/gpu/arm,mali-bifrost.yaml

@@ -69,6 +69,8 @@ properties:
 
       where voltage is in V, frequency is in MHz.
 
+  dma-coherent: true
+
 required:
   - compatible
   - reg