dt-bindings: i2c: rcar: Document r8a77961 support

Document support for the I2C controller in the Renesas R-Car M3-W+
(R8A77961) SoC.

Update all references to R-Car M3-W from "r8a7796" to "r8a77960", to
avoid confusion between R-Car M3-W (R8A77960) and M3-W+.

No driver update is needed.

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