dts: bindings: adc: add properties for stm32 adc clock source

Add two properties to define the STM32 ADC clock source:
- Clock source: synchronous or asynchronous
- Clock prescaler
By combining these two parameters, it will be possible to set the desired
ADC clock for most series.

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