dt-bindings: i2c: add binding for Socionext SynQuacer I2C

Add a binding for the I2C controller that can be found in the Socionext
SynQuacer SoC.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>