dt-bindings: iio: stm32-adc: add generic channel binding

Add ADC generic channel binding. This binding should
be used as an alternate to legacy channel properties
whenever possible.
ADC generic channel binding allows to identify supported
internal channels through the following reserved label names:
"vddcore", "vrefint" and "vbat".
This binding also allows to set a different sampling time
for each channel.

Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Reviewed-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211014131228.4692-2-olivier.moysan@foss.st.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>