dt-bindings: iio: adc: stm32: add optional st,min-sample-time-nsecs

STM32 ADC allows each channel to be sampled with a different sampling
time. There's an application note that deals with this: 'How to get
the best ADC accuracy in STM32...' It basically depends on analog input
signal electrical properties (depends on board).

Add optional 'st,min-sample-time-nsecs' property so this can be tuned
in dt.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>