i2c: stm32f7: add PM_SLEEP suspend/resume support

Backup/restore I2C registers as part of the suspend/resume
handlers. The device is marked as suspended to ensure that
transfers are rejected during the suspended period.

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