dt-bindings: i2c-stm32: SYSCFG Fast Mode Plus support for I2C STM32F7

Append optional bindings to update SYSCFG Fast Mode Plus bits if
Fast Mode Plus speed is selected.

Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>