dt-bindings: i2c: Document I2C controller binding for MT6797 SoC

I2C controller driver for MT6577 SoC is reused for MT6797 SoC. Hence,
document that in DT binding.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Wolfram Sang <wsa@kernel.org>
Link: https://lore.kernel.org/r/20200222162444.11590-2-manivannan.sadhasivam@linaro.org


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>