drivers: sensor: add a stm32 sensor driver for the internal VBat

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>