i2c: rcar: improve accuracy for R-Car Gen3+

With some new registers, SCL can be calculated to be closer to the
desired rate. Apply the new formula for R-Car Gen3 device types.

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