Add MT8173 I2C device nodes, include I2C controllers and pins.
MT8173 has six I2C controllers, from i2c0 to i2c6, exclude i2c5.
The 6th I2C controller register base doesn't next to 5th I2C,
and there is a hardware between 5th and 6th I2C controller. So
SoC designer name 6th controller as "i2c6", not "i2c5".

Signed-off-by: Eddie Huang <eddie.huang@mediatek.com>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>