dt-bindings:iio:adc: add generic settling-time-us and oversampling-ratio channel properties

Settling time and over sampling is a typical challenge for different IIO ADC
devices. So, introduce channel specific settling-time-us and oversampling-ratio
properties to cover this use case.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210428073208.19570-2-o.rempel@pengutronix.de


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