i2c: sh_mobile: Add support for r8a73a4 and sh73a0

Add support for r8a73a4 (R-Mobile APE6) and sh73a0 (SH-Mobile AG5).
On these SoCs, the operating clock runs faster that on previous SoCs,
and the internal SCL clock counter gets incremented every 2 clocks of
the operating clock, just like on R-Car Gen2.

Cfr. the "/2" in the calculation of ICCL/ICCH in section "I2C Bus
Interface (IIC)", subsection "Transfer Rate" of the datasheets.

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