adc: stm32mp15: add support of generic channels binding

Add support of generic IIO channels binding:
./devicetree/bindings/iio/adc/adc.yaml
Keep support of st,adc-channels for backward compatibility.

Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>