dt-bindings: i2c-stm32: Document the STM32F7 I2C bindings

This patch adds the documentation of device tree bindings for STM32F7 I2C

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