dts: bindings: stm32 adc has a Vbat channel monitoring

Like other temperature or vrefint sensor, the stm32 mcu
also have a Vbat monitoring internal channels on ADC.
Add this entry to the device tree.
The vref is usually 3300mV present on the target board.

Signed-off-by: Francois Ramu <francois.ramu@st.com>