i2c: rk3x: adjust the LOW divison based on characteristics of SCL

As show in I2C specification:
- Standard-mode: the minimum HIGH period of the scl clock is 4.0us
                 the minimum LOW period of the scl clock is 4.7us
- Fast-mode: the minimum HIGH period of the scl clock is 0.6us
             the minimum LOW period of the scl clock is 1.3us

I have measured i2c SCL waveforms in fast-mode by oscilloscope
on rk3288-pinky board. the LOW period of the scl clock is 1.3us.
It is so critical that we must adjust LOW division to increase
the LOW period of the scl clock.

Thanks Doug for the suggestion about division formulas.

Signed-off-by: Addy Ke <addy.ke@rock-chips.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Max Schwarz <max.schwarz@online.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>