i2c: stm32f7: SYSCFG Fast Mode Plus support for I2C STM32F7

Read SYSCFG bindings to set Fast Mode Plus bits if Fast Mode Plus
speed is selected.

Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>