i2c: mediatek: send i2c master code at 400k

The speed of sending i2c master code in high-speed mode depends on
source clock, clock-div and TIMING register. The source clock and
clock-div of different SoC are not all the same. In order to send
i2c master code at 400k in high-speed mode, a appropriate value
should be set to TIMING register for a certain source clock and
clock-div.

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