dt-bindings: i2c-mtk: Add mt7623 binding

The mt7623 dtsi has support for the i2c block, but this is not documented.
Add the documentation for SoC mt7623 to de description.

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