For STM32G4, ADC clock frequency should be equal or less than 60MHz.
To satisfy this specification, ADC clock prescaler should be equal or
greater than 4 (For example, NUCLEO_G474RE runs 170MHz).

In addition, to obtain accurate internal channel value,
the ADC clock prescaler is set to 16 because vbat needs at least 12us
(16/170*247.5=23.3us).
Signed-off-by: NYuuki NAGAO <wf.yn386@gmail.com>