dt-bindings: i2c: i2c-mt65xx: Document MediaTek MT6878 I2C

Document the I2C controllers found in the MediaTek MT6878 SoC, by adding
a new compatible string for the controllers. Their design is compatible
with the design from the MediaTek MT8188 SoC.

Signed-off-by: Igor Belwon <igor.belwon@mentallysanemainliners.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20250920-mt6878-i2c-bringup-v2-1-70a951f10be9@mentallysanemainliners.org


Signed-off-by: Andi Shyti <andi.shyti@kernel.org>