iio: adc: stm32-adc: use generic binding for sample-time

iio: adc: stm32-adc: use generic binding for sample-time

Add st,min-sample-time-nsecs to channel generic binding.
Sample time can be defined par channel node. If a channel
is configured as differential, the same sample time applies
for both inputs.
Keep support of legacy st,min-sample-time-nsecs property
for backward compatibility.

Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Reviewed-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/20211014131228.4692-8-olivier.moysan@foss.st.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>