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