drivers: i2c: stm32 driver has get_config api

Add the get_config API to the stm32 I2C driver.
It will return the value of the Clock  Control register for i2C V1 bus
or the TIMING register for the I2C V2 bus.
This is hold by a i2c_config_timing structure of the device data
Add a bool to check if the I2C is configured or not.

Signed-off-by: Francois Ramu <francois.ramu@st.com>