dts: bindings: adc: stm32: add oversampler

On STM32 ADC, there are currently two types of oversamplers (if present),
one with 8 available oversampling values, the other with 1024.
To simplify the driver, add the oversampler as a dts property.
Also add defines to avoid magic values in the dtsi.

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