dt-bindings: i2c: sh_mobile: Document r8a77961 support

Document support for the IIC controller in the Renesas R-Car M3-W+
(R8A77961) SoC.

No driver update is needed.

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