dt-bindings: i2c: i2c-mtk: add support for MT8516

Add binding documentation of i2c-mtk for MT8516 SoC.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>