iio: adc: hx711: Add DT binding for avia,hx711

Add DT bindings for avia,hx711
Add vendor avia to vendor list

Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>