iio: twl6030-gpadc: TWL6030, TWL6032 GPADC driver

The GPADC is general purpose ADC found on TWL6030, and TWL6032 PMIC,
known also as Phoenix and PhoenixLite.

The TWL6030 and TWL6032 have GPADC with 17 and 19 channels
respectively. Some channels have current source and are used for
measuring voltage drop on resistive load for detecting battery ID
resistance, or measuring voltage drop on NTC resistors for external
temperature measurements. Some channels measure voltage, (i.e. battery
voltage), and have voltage dividers, thus, capable to scale voltage.
Some channels are dedicated for measuring die temperature.

Some channels are calibrated in 2 points, having offsets from ideal
values kept in trim registers. This is used to correct measurements.

The differences between GPADC in TWL6030 and TWL6032:
- 10 bit vs 12 bit ADC;
- 17 vs 19 channels;
- channels have different purpose(i.e. battery voltage
  channel 8 vs channel 18);
- trim values are interpreted differently.

Based on the driver patched from Balaji TK, Graeme Gregory, Ambresh K,
Girish S Ghongdemath.

Signed-off-by: Balaji T K <balajitk@ti.com>
Signed-off-by: Graeme Gregory <gg@slimlogic.co.uk>
Signed-off-by: Oleksandr Kozaruk <oleksandr.kozaruk@ti.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/adc/Kconfig

@@ -171,6 +171,20 @@ config TI_AM335X_ADC
 	  Say yes here to build support for Texas Instruments ADC
 	  driver which is also a MFD client.
 
+config TWL6030_GPADC
+	tristate "TWL6030 GPADC (General Purpose A/D Converter) Support"
+	depends on TWL4030_CORE
+	default n
+	help
+	  Say yes here if you want support for the TWL6030/TWL6032 General
+	  Purpose A/D Converter. This will add support for battery type
+	  detection, battery voltage and temperature measurement, die
+	  temperature measurement, system supply voltage, audio accessory,
+	  USB ID detection.
+
+	  This driver can also be built as a module. If so, the module will be
+	  called twl6030-gpadc.
+
 config VIPERBOARD_ADC
 	tristate "Viperboard ADC support"
 	depends on MFD_VIPERBOARD && USB

drivers/iio/adc/Makefile

@@ -19,4 +19,5 @@ obj-$(CONFIG_MCP320X) += mcp320x.o
 obj-$(CONFIG_NAU7802) += nau7802.o
 obj-$(CONFIG_TI_ADC081C) += ti-adc081c.o
 obj-$(CONFIG_TI_AM335X_ADC) += ti_am335x_adc.o
+obj-$(CONFIG_TWL6030_GPADC) += twl6030-gpadc.o
 obj-$(CONFIG_VIPERBOARD_ADC) += viperboard_adc.o