avia-hx711.yaml: transform DT binding to YAML

replace avia-hx711.txt by avia-hx711.yaml as yaml devicetree
documentation for avia hx711 iio adc sensor

Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>