dt-bindings: Document the STM32 I2C bindings

This patch adds documentation of device tree bindings for the STM32 I2C
controller.

Signed-off-by: M'boumba Cedric Madianga <cedric.madianga@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>