ARM: dts: stm32: Update to generic ADC channel binding on DHSOM systems

The generic ADC channel binding is recommended over legacy one, update the
DT to the modern binding. No functional change. For further details, see
commit which adds the generic binding to STM32 ADC binding document:
'664b9879 ("dt-bindings: iio: stm32-adc: add generic channel binding")'

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>