iio: adc: hx711: Add IIO driver for AVIA HX711

This is the IIO driver for AVIA HX711 ADC which is mostly used in weighting
cells.

The protocol is quite simple and using GPIOs:
One GPIO is used as clock (SCK) while another GPIO is read (DOUT)

The raw value read from the chip is delivered.
To get a weight one needs to subtract the zero offset and scale it.
Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/adc/Kconfig

@@ -247,6 +247,25 @@ config HI8435
 	  This driver can also be built as a module. If so, the module will be
 	  called hi8435.
 
+config HX711
+	tristate "AVIA HX711 ADC for weight cells"
+	depends on GPIOLIB
+	help
+	  If you say yes here you get support for AVIA HX711 ADC which is used
+	  for weigh cells
+
+	  This driver uses two GPIOs, one acts as the clock and controls the
+	  channel selection and gain, the other one is used for the measurement
+          data
+
+	  Currently the raw value is read from the chip and delivered.
+	  To get an actual weight one needs to subtract the
+	  zero offset and multiply by a scale factor.
+	  This should be done in userspace.
+
+	  This driver can also be built as a module. If so, the module will be
+	  called hx711.
+
 config INA2XX_ADC
 	tristate "Texas Instruments INA2xx Power Monitors IIO driver"
 	depends on I2C && !SENSORS_INA2XX

drivers/iio/adc/Makefile

@@ -25,6 +25,7 @@ obj-$(CONFIG_ENVELOPE_DETECTOR) += envelope-detector.o
 obj-$(CONFIG_EXYNOS_ADC) += exynos_adc.o
 obj-$(CONFIG_FSL_MX25_ADC) += fsl-imx25-gcq.o
 obj-$(CONFIG_HI8435) += hi8435.o
+obj-$(CONFIG_HX711) += hx711.o
 obj-$(CONFIG_IMX7D_ADC) += imx7d_adc.o
 obj-$(CONFIG_INA2XX_ADC) += ina2xx-adc.o
 obj-$(CONFIG_LP8788_ADC) += lp8788_adc.o