dt-bindings: display: mediatek: ovl: Add compatible for MediaTek MT6795

Add a compatible string for MediaTek Helio X10 MT6795's OVL block: this
is the same as MT8173.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20230412112739.160376-7-angelogioacchino.delregno@collabora.com/


Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>