i2c: sh_mobile: use new clock calculation formulas for Gen2

We measured the clock on a Lager and an Ebisu board. The new formula
gives better results for both. So after Gen3, switch to this formula for
all Gen2 SoCs.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>