dt-bindings: i2c: rcar: Add r8a774b1 support

Document RZ/G2N (R8A774B1) I2C compatibility with the relevant driver
dt-bindings.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>