dt-bindings: gpu: mali-bifrost: Document nvmem for speedbin support

Some SoCs implementing ARM Mali GPUs may be subject to speed binning
and the usable bin is read from nvmem: document the addition of nvmem
and nvmem-cells for 'speed-bin'.
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230323090822.61766-2-angelogioacchino.delregno@collabora.com

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

@@ -104,6 +104,13 @@ properties:
 
   dma-coherent: true
 
+  nvmem-cell-names:
+    items:
+      - const: speed-bin
+
+  nvmem-cells:
+    maxItems: 1
+
 required:
   - compatible
   - reg