dm: adc: add uclass's mask and conversion helpers

Add two functions to ADC uclass's:
- adc_raw_to_uV() to ease ADC raw value conversion to microvolts
- adc_channel_mask() to get channels on consumer side

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>