dt-bindings:iio:adc:holt,hi8435 yaml conversion

Very simple binding.  I've changed the example to use the node
name threshold-detector@0 as sensor@0 seemed too generic.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Vladimir Barinov <vladimir.barinov@cogentembedded.com>
Link: https://lore.kernel.org/r/20200909175946.395313-18-jic23@kernel.org