Get the vref-mv through the ADC API instead of the device tree. This
saves 8 bytes of SRAM and will be future proof in case, one day, support
for a variable vref is added.
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Similar to the internal temperature sensor of the stm32
this driver controls the Vbat monitoring in Volts,
using an ADC internal input and the stm32-vbat node of the DTS.
The ref voltage is given by the ADC.
Note that stm32F1x does not propose the feature.
Signed-off-by: Francois Ramu <francois.ramu@st.com>