cmd: adc: add an option to scan some or all available channels

Add new option to 'adc' command to do a single scan of:
- some channel(s), using mask argument
- all channels available on an ADC device (when optional mask is omitted).

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