i2c: rcar: document R8A77970 bindings

R-Car V3M (R8A77970) SoC also has the R-Car gen3 compatible I2C controller,
so document  the SoC specific bindings.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.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>