stm32f3: i2c support removed from the i2c_common_all files because f3 is too different.
stm32f3: i2c support now in f3 directory.

Very few functions can be shared (~ 3). A possible solutions is to move
i2c_common_all to i2c_common_f124, create i2c_common_all and move f3/i2c to i2c_common_f3. Who agrees?
stm32f3: Some additional f3 clock functions for the i2c.