i2c: mediatek: Add i2c compatible for MediaTek MT2712

Add i2c compatible for MT2712. Compare to MT8173 i2c controller,
internal divider of i2c source clock need to be configured for
MT2712 i2c speed calculation.

Signed-off-by: Jun Gao <jun.gao@mediatek.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>