drivers: i2c: stm32: PM device support

Clock and pins used by the I2C device are suspended when power
manager requires it.
Do not compile function i2c_stm32_suspend when PM_DEVICE isn't
 enabled as it is left unused and will make the compiler throw
 a warning.

Signed-off-by: Cyril Fougeray <cyril.fougeray@worldcoin.org>