dt-bindings: display: mediatek, ovl: Fix 'iommu' required property typo

The property is called 'iommus' and not 'iommu'. Fix this typo.

Fixes: 4ed545e7 ("dt-bindings: display: mediatek: disp: split each block to individual yaml")
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: jason-jh.lin <jason-jh.lin@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220309134702.9942-4-jason-jh.lin@mediatek.com