dt-bindings: i2c: renesas,riic: Convert to json-schema

Convert the Renesas RZ/A I2C Bus Interface (RIIC) Device Tree binding
documentation to json-schema.

Document missing properties.
Update the example to match reality.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>