i2c: sh_mobile: use new clock calculation formulas for Gen3

We could finally measure the clock on an Ebisu board. The new formula
gives way better results, i.e. 100kHz instead of 106kHz and 400kHz
instead of 387kHz. Switch to these formulas for all Gen3 SoCs.

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