dt-bindings: i2c: Add MediaTek MT2712 i2c binding

Add MT2712 i2c binding to binding file. Compare to MT8173 i2c
controller, MT2712 has timing adjust registers which can adjust
the internal divider of i2c source clock, SCL duty cycle, SCL
compare point, start(repeated start) and stop time, SDA change
time.

Signed-off-by: Jun Gao <jun.gao@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>