arm64: dts: mediatek: mt8186: Change gpu speedbin nvmem cell name

MT8186's GPU speedbin value must be interpreted, or the value will not
be meaningful.
Use the correct "gpu-speedbin" nvmem cell name for the GPU speedbin to
allow triggering the cell info fixup handler, hence feeding the right
speedbin number to the users.

Cc: stable@vger.kernel.org
Fixes: 263d2fd0 ("arm64: dts: mediatek: mt8186: Add GPU speed bin NVMEM cells")
Reviewed-by: Chen-Yu Tsai <wenst@chromium.org>
Link: https://lore.kernel.org/r/20231005151150.355536-1-angelogioacchino.delregno@collabora.com


Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>