dt-bindings: iio: io-channel-mux: Add property for settle time

Hardware may require some time for the muxed analog signals to settle
after the muxing is changed.  Allow this time to be specified in the
devicetree.

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Tested-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Peter Rosin <peda@axentia.se>
Link: https://lore.kernel.org/r/20211007134641.13417-3-vincent.whitchurch@axis.com


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