hx71x: Load Cell Skeleton and HX71x bulk ADC

* Create the load_cell host module skeleton to create the sensors and start taking samples.
* Add support for the HX717 and HX711 ADC sensors.

Signed-off-by: Gareth Farrington <gareth@waves.ky>

docs/Config_Reference.md

@@ -4645,6 +4645,63 @@ adc2:
 #   above parameters.
 ```
 
+## Load Cells
+
+### [load_cell]
+Load Cell. Uses an ADC sensor attached to a load cell to create a digital
+scale.
+
+```
+[load_cell]
+sensor_type:
+#   This must be one of the supported Sensor types, see `Load Cell Sensors`
+```
+
+### Load Cell Sensors
+
+Load Cell supports dedicated Analog to Digital Converter (ADC) chips that sample
+at a high data rate.
+
+#### XH711
+This is a 24 bit low sample rate chip using "bit-bang" communications. It is
+suitable for filament scales.
+```
+sensor_type: hx711
+sclk_pin:
+#   The pin connected to the HX711 clock line. This parameter must be provided.
+dout_pin:
+#   The pin connected to the HX711 data output line. This parameter must be
+#   provided.
+#gain: A-128
+#   Valid values for gain are: A-128, A-64, B-32. The default is A-128.
+#   'A' denotes the input channel and the number denotes the gain. Only the 3
+#   listed combinations are supported by the chip. Note that changing the gain
+#   setting also selects the channel being read.
+#sample_rate: 80
+#   Valid values for sample_rate are 80 or 10. The default value is 80.
+#   This must match the wiring of the chip. The sample rate cannot be changed
+#   in software.
+```
+
+### HX717
+This is the 4x higher sample rate version of the HX711, suitable for probing.
+```
+sensor_type: hx717
+sclk_pin:
+#   The pin connected to the HX717 clock line. This parameter must be provided.
+dout_pin:
+#   The pin connected to the HX717 data output line. This parameter must be
+#   provided.
+#gain: A-128
+#   Valid values for gain are A-128, B-64, A-64, B-8.
+#   'A' denotes the input channel and the number denotes the gain setting.
+#   Only the 4 listed combinations are supported by the chip. Note that
+#   changing the gain setting also selects the channel being read.
+#sample_rate: 320
+#   Valid values for sample_rate are: 10, 20, 80, 320. The default is 320.
+#   This must match the wiring of the chip. The sample rate cannot be changed
+#   in software.
+```
 ## Board specific hardware support
 
 ### [sx1509]