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

Documentation/devicetree/bindings/iio/adc/adc.yaml

@@ -39,4 +39,16 @@ properties:
       The first value specifies the positive input pin, the second
       specifies the negative input pin.
 
+  settling-time-us:
+    description:
+      Time between enabling the channel and first stable readings.
+
+  oversampling-ratio:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description:
+      Oversampling is used as replacement of or addition to the low-pass filter.
+      In some cases, the desired filtering characteristics are a function the
+      device design and can interact with other characteristics such as
+      settling time.
+
 additionalProperties: true