dts: bindings: adc: add adc bindings for stm32n6

STM32N6 ADC doesn't have an integrated prescaler, so these properties have
to be removed.
Though the F1 compatible also removes these two same properties, grouping
the N6 with the F1 would not be convenient since there are lots of
differences between the two.

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