dt-bindings: i2c: document R8A77965 bindings

R-Car M3-N (R8A77965) SoC has a R-Car Gen3-compatible I2C controller.

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>