dt-bindings: i2c: rcar: Document r8a7744 support

Document i2c Device Tree support for RZ/G1N (R8A7744) SoC, which is
compatible with R-Car Gen2 SoC family.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>