iio: hx711: add clock-frequency property in DT

Add clock-frequency property for hx711 ADC. This is the frequency of
PD_SCK.

After PD_SCK goes high DOUT is read just before PD_SCK goes down again.
This is necessary because of parasitic capacitance on the wiring.
Signed-off-by: NAndreas Klinger <ak@it-klinger.de>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NJonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/adc/avia-hx711.txt

@@ -8,11 +8,17 @@ Required properties:
 		See Documentation/devicetree/bindings/gpio/gpio.txt
  - avdd-supply:	Definition of the regulator used as analog supply
 
+Optional properties:
+ - clock-frequency:	Frequency of PD_SCK in Hz
+			Minimum value allowed is 10 kHz because of maximum
+			high time of 50 microseconds.
+
 Example:
 weight@0 {
 	compatible = "avia,hx711";
 	sck-gpios = <&gpio3 10 GPIO_ACTIVE_HIGH>;
 	dout-gpios = <&gpio0 7 GPIO_ACTIVE_HIGH>;
 	avdd-suppy = <&avdd>;
+	clock-frequency = <100000>;
 };