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 <[e-mail]>
Acked-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: Boris Brezillon <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

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

···
 
   dma-coherent: true
 
+  nvmem-cell-names:
+    items:
+      - const: speed-bin
+
+  nvmem-cells:
+    maxItems: 1
+
 required:
   - compatible
   - reg