dts: bindings: adc: Add bindings for internal STM32 ADC channels

Add some bindings to define STM32 ADC internal channels for:
 - Temperature
 - Vref
 - Vbat
The goal is to transfer information to the dts instead of inside the
ADC driver.

Signed-off-by: Guillaume Gautier <guillaume.gautier-ext@st.com>